diff --git a/docs/source/io.rst b/docs/source/io.rst
index 100ccb1..baea14b 100644
--- a/docs/source/io.rst
+++ b/docs/source/io.rst
@@ -33,7 +33,7 @@ These generic functions are less flexible than those first mentioned, and theref
       model = rn.pmodel.InterpolationModel(...)
       model.add_dof_from_files(..., file_format = "outcar")
 
-:meth:`.InterpolationModel.add_dof_from_files` and other methods like it rely on these generic methods, as apparent from the ``file_format`` argument.
+:meth:`.InterpolationModel.add_dof_from_files` and other methods like it rely on these generic methods, as apparent from the ``file_format`` argument. When ``file_format=="auto"``, ramannoodle will guess the file format based on the filename. Otherwise, ``file_format`` should be set according to the next section.
 
 .. _Supported formats:
 
diff --git a/ramannoodle/io/generic.py b/ramannoodle/io/generic.py
index 3c81f8b..a90dcac 100644
--- a/ramannoodle/io/generic.py
+++ b/ramannoodle/io/generic.py
@@ -18,6 +18,7 @@
 from ramannoodle.structure._reference import ReferenceStructure
 from ramannoodle.exceptions import UserError, get_torch_missing_error
 import ramannoodle.io.vasp as vasp_io
+from ramannoodle.io._utils import pathify_as_list
 
 TORCH_PRESENT = True
 try:
@@ -66,15 +67,44 @@
 _TRAJECTORY_WRITERS = {"xdatcar": vasp_io.xdatcar.write_trajectory}
 
 
-def read_phonons(filepath: str | Path, file_format: str) -> Phonons:
+def _process_file_format(filepaths: str | Path | list[str] | list[Path]) -> str:
+    """Guess file format from filepath(s).
+
+    The implementation is very simple. It simply looks for relevant
+    strings in the filename. In the case of multiple files, the first
+    file is used to guess the file format.
+
+    Parameters
+    ----------
+    filepath
+
+    Returns
+    -------
+    :
+        Lowercase file_format string.
+    """
+    filepaths = pathify_as_list(filepaths)
+    filename = filepaths[0].name.lower()
+    if "outcar" in filename:
+        return "outcar"
+    if "vasprun.xml" in filename or "vasprun" in filename:
+        return "vasprun.xml"
+    if "xdatcar" in filename:
+        return "xdatcar"
+    if "poscar" in filename:
+        return "poscar"
+    raise ValueError(f"could not guess file format: {filepaths[0]}")
+
+
+def read_phonons(filepath: str | Path, file_format: str = "auto") -> Phonons:
     """Read phonons from a file.
 
     Parameters
     ----------
     filepath
     file_format
-        Supports ``"outcar"``, ``"vasprun.xml"`` (see :ref:`Supported formats`). Not
-        case sensitive.
+        Supports ``"outcar"``, ``"vasprun.xml"``, and ``"auto"`` (see :ref:`Supported
+        formats`). Not case sensitive.
 
     Returns
     -------
@@ -87,22 +117,24 @@ def read_phonons(filepath: str | Path, file_format: str) -> Phonons:
     InvalidFileException
         Invalid file.
     """
+    if file_format.lower() == "auto":
+        file_format = _process_file_format(filepath)
     try:
         return _PHONON_READERS[file_format.lower()](filepath)
     except KeyError as exc:
         raise ValueError(f"unsupported format: {file_format}") from exc
 
 
-def read_trajectory(filepath: str | Path, file_format: str) -> Trajectory:
+def read_trajectory(filepath: str | Path, file_format: str = "auto") -> Trajectory:
     """Read molecular dynamics trajectory from a file.
 
     Parameters
     ----------
     filepath
     file_format
-        Supports ``"outcar"``, ``"vasprun.xml"``, (see :ref:`Supported formats`). Not
-        case sensitive. Use :func:`.vasp.xdatcar.read_trajectory` to read a trajectory
-        from an XDATCAR.
+        Supports ``"outcar"``, ``"vasprun.xml"``, and ``"auto"`` (see :ref:`Supported
+        formats`). Not case sensitive. Use :func:`.vasp.xdatcar.read_trajectory` to
+        read a trajectory from an XDATCAR.
 
     Returns
     -------
@@ -115,6 +147,8 @@ def read_trajectory(filepath: str | Path, file_format: str) -> Trajectory:
     InvalidFileException
         Invalid file.
     """
+    if file_format.lower() == "auto":
+        file_format = _process_file_format(filepath)
     try:
         return _TRAJECTORY_READERS[file_format.lower()](filepath)
     except KeyError as exc:
@@ -127,7 +161,7 @@ def read_trajectory(filepath: str | Path, file_format: str) -> Trajectory:
 
 def read_positions_and_polarizability(
     filepath: str | Path,
-    file_format: str,
+    file_format: str = "auto",
 ) -> tuple[NDArray[np.float64], NDArray[np.float64]]:
     """Read fractional positions and polarizability from a file.
 
@@ -135,8 +169,8 @@ def read_positions_and_polarizability(
     ----------
     filepath
     file_format
-        Supports ``"outcar"``, ``"vasprun.xml"`` (see :ref:`Supported formats`). Not
-        case sensitive.
+        Supports ``"outcar"``, ``"vasprun.xml"``, and ``"auto"`` (see :ref:`Supported
+        formats`). Not case sensitive.
 
     Returns
     -------
@@ -152,6 +186,8 @@ def read_positions_and_polarizability(
     InvalidFileException
         Invalid file.
     """
+    if file_format.lower() == "auto":
+        file_format = _process_file_format(filepath)
     try:
         return _POSITION_AND_POLARIZABILITY_READERS[file_format.lower()](filepath)
     except KeyError as exc:
@@ -160,7 +196,7 @@ def read_positions_and_polarizability(
 
 def read_structure_and_polarizability(
     filepath: str | Path,
-    file_format: str,
+    file_format: str = "auto",
 ) -> tuple[NDArray[np.float64], list[int], NDArray[np.float64], NDArray[np.float64]]:
     """Read lattice, atomic numbers, fractional positions, polarizability from a file.
 
@@ -168,8 +204,8 @@ def read_structure_and_polarizability(
     ----------
     filepath
     file_format
-        Supports ``"outcar"``, ``"vasprun.xml"`` (see :ref:`Supported formats`). Not
-        case sensitive.
+        Supports ``"outcar"``, ``"vasprun.xml"``, and ``"auto"`` (see :ref:`Supported
+        formats`). Not case sensitive.
 
     Returns
     -------
@@ -187,6 +223,8 @@ def read_structure_and_polarizability(
     InvalidFileException
         Invalid file.
     """
+    if file_format.lower() == "auto":
+        file_format = _process_file_format(filepath)
     try:
         return _STRUCTURE_AND_POLARIZABILITY_READERS[file_format.lower()](filepath)
     except KeyError as exc:
@@ -195,7 +233,7 @@ def read_structure_and_polarizability(
 
 def read_polarizability_dataset(
     filepaths: str | Path | list[str] | list[Path],
-    file_format: str,
+    file_format: str = "auto",
 ) -> "PolarizabilityDataset":
     """Read polarizability dataset from files.
 
@@ -203,8 +241,8 @@ def read_polarizability_dataset(
     ----------
     filepaths
     file_format
-        Supports ``"outcar"``, ``"vasprun.xml"`` (see :ref:`Supported formats`). Not
-        case sensitive.
+        Supports ``"outcar"``, ``"vasprun.xml"``, and ``"auto"`` (see :ref:`Supported
+        formats`). Not case sensitive.
 
     Returns
     -------
@@ -221,6 +259,8 @@ def read_polarizability_dataset(
     """
     if not TORCH_PRESENT:
         raise get_torch_missing_error()
+    if file_format.lower() == "auto":
+        file_format = _process_file_format(filepaths)
     try:
         return _POLARIZABILITY_DATASET_READERS[file_format.lower()](filepaths)
     except KeyError as exc:
@@ -229,7 +269,7 @@ def read_polarizability_dataset(
 
 def read_positions(
     filepath: str | Path,
-    file_format: str,
+    file_format: str = "auto",
 ) -> NDArray[np.float64]:
     """Read fractional positions from a file.
 
@@ -237,8 +277,8 @@ def read_positions(
     ----------
     filepath
     file_format
-        Supports ``"outcar"``, ``"poscar"``, ``"xdatcar"``, ``"vasprun.xml"``  (see
-        :ref:`Supported formats`). Not case sensitive.
+        Supports ``"outcar"``, ``"poscar"``, ``"xdatcar"``, ``"vasprun.xml"``, and
+        ``"auto"`  (see :ref:`Supported formats`). Not case sensitive.
 
     Returns
     -------
@@ -253,21 +293,25 @@ def read_positions(
         Invalid file.
 
     """
+    if file_format.lower() == "auto":
+        file_format = _process_file_format(filepath)
     try:
         return _POSITION_READERS[file_format.lower()](filepath)
     except KeyError as exc:
         raise ValueError(f"unsupported format: {file_format}") from exc
 
 
-def read_ref_structure(filepath: str | Path, file_format: str) -> ReferenceStructure:
+def read_ref_structure(
+    filepath: str | Path, file_format: str = "auto"
+) -> ReferenceStructure:
     """Read reference structure from a file.
 
     Parameters
     ----------
     filepath
     file_format
-        Supports ``"outcar"``, ``"poscar"``, ``"xdatcar"``, ``"vasprun.xml"`` (see
-        :ref:`Supported formats`).
+        Supports ``"outcar"``, ``"poscar"``, ``"xdatcar"``, ``"vasprun.xml"``, and
+        ``"auto"`` (see :ref:`Supported formats`).
 
     Returns
     -------
@@ -282,6 +326,8 @@ def read_ref_structure(filepath: str | Path, file_format: str) -> ReferenceStruc
     SymmetryException
         Structural symmetry determination failed.
     """
+    if file_format.lower() == "auto":
+        file_format = _process_file_format(filepath)
     try:
         return _REFERENCE_STRUCTURE_READERS[file_format.lower()](filepath)
     except KeyError as exc:
@@ -293,7 +339,7 @@ def write_structure(  # pylint: disable=too-many-arguments,too-many-positional-a
     atomic_numbers: list[int],
     positions: NDArray[np.float64],
     filepath: str | Path,
-    file_format: str,
+    file_format: str = "auto",
     overwrite: bool = False,
 ) -> None:
     """Write structure to file.
@@ -308,7 +354,8 @@ def write_structure(  # pylint: disable=too-many-arguments,too-many-positional-a
         (fractional) Array with shape (N,3).
     filepath
     file_format
-        Supports ``"poscar"`` (see :ref:`Supported formats`). Not case sensitive.
+        Supports ``"poscar"`` and ``"auto"`` (see :ref:`Supported formats`). Not case
+        sensitive.
     overwrite
         Overwrite the file if it exists.
     label
@@ -319,6 +366,8 @@ def write_structure(  # pylint: disable=too-many-arguments,too-many-positional-a
     FileExistsError
         File exists and ``overwrite == False``.
     """
+    if file_format.lower() == "auto":
+        file_format = _process_file_format(filepath)
     try:
         _STRUCTURE_WRITERS[file_format.lower()](
             lattice=lattice,
@@ -337,7 +386,7 @@ def write_trajectory(
     atomic_numbers: list[int],
     positions_ts: NDArray[np.float64],
     filepath: str | Path,
-    file_format: str,
+    file_format: str = "auto",
     overwrite: bool = False,
 ) -> None:
     """Write trajectory to file.
@@ -353,7 +402,8 @@ def write_trajectory(
         configurations.
     filepath
     file_format
-        Supports ``"xdatcar"`` (see :ref:`Supported formats`). Not case sensitive.
+        Supports ``"xdatcar"`` and ``"auto"`` (see :ref:`Supported formats`). Not case
+        sensitive.
     overwrite
         Overwrite the file if it exists.
 
@@ -362,6 +412,8 @@ def write_trajectory(
     FileExistsError
         File exists and ``overwrite == False``.
     """
+    if file_format.lower() == "auto":
+        file_format = _process_file_format(filepath)
     try:
         _TRAJECTORY_WRITERS[file_format.lower()](
             lattice=lattice,
diff --git a/ramannoodle/pmodel/_art.py b/ramannoodle/pmodel/_art.py
index 1162c6b..b3f94b8 100644
--- a/ramannoodle/pmodel/_art.py
+++ b/ramannoodle/pmodel/_art.py
@@ -199,7 +199,7 @@ def add_art(
     def add_art_from_files(
         self,
         filepaths: str | Path | list[str] | list[Path],
-        file_format: str,
+        file_format: str = "auto",
     ) -> None:
         """Add an atomic Raman tensor (ART) from file(s).
 
@@ -211,9 +211,9 @@ def add_art_from_files(
         ----------
         filepaths
         file_format
-            Supports ``"outcar"`` and ``"vasprun.xml"``. If dummy model, supports
-            ``"poscar"`` and ``"xdatcar"`` as well (see :ref:`Supported formats`). Not
-            case sensitive.
+            Supports ``"outcar"``, ``"vasprun.xml"``, and ``"auto"``. If dummy model,
+            supports ``"poscar"`` and ``"xdatcar"`` as well (see :ref:`Supported
+            formats`). Not case sensitive.
 
         Raises
         ------
diff --git a/ramannoodle/pmodel/_interpolation.py b/ramannoodle/pmodel/_interpolation.py
index 9d1a434..3a405e5 100644
--- a/ramannoodle/pmodel/_interpolation.py
+++ b/ramannoodle/pmodel/_interpolation.py
@@ -624,7 +624,9 @@ def add_dof_from_pymatgen(
         )
 
     def _read_dof(
-        self, filepaths: str | Path | list[str] | list[Path], file_format: str
+        self,
+        filepaths: str | Path | list[str] | list[Path],
+        file_format: str,
     ) -> tuple[NDArray[np.float64], NDArray[np.float64], NDArray[np.float64]]:
         """Read displacements, amplitudes, and polarizabilities from file(s).
 
diff --git a/ramannoodle/structure/_displace.py b/ramannoodle/structure/_displace.py
index ff446a2..7ee13b0 100644
--- a/ramannoodle/structure/_displace.py
+++ b/ramannoodle/structure/_displace.py
@@ -80,7 +80,7 @@ def write_displaced_structures(
     cart_displacement: NDArray[np.float64],
     amplitudes: NDArray[np.float64],
     filepaths: str | Path | list[str] | list[Path],
-    file_format: str,
+    file_format: str = "auto",
     overwrite: bool = False,
 ) -> None:
     """Write displaced structures to files.
@@ -96,7 +96,8 @@ def write_displaced_structures(
         (Å) Array with shape (M,).
     filepaths
     file_format
-        Supports ``"poscar"`` (see :ref:`Supported formats`). Not case sensitive.
+        Supports ``"poscar"`` and ``"auto"`` (see :ref:`Supported formats`). Not case
+        sensitive.
     overwrite
         If ``True``, overwrite the file if it exists.
     """
@@ -163,7 +164,7 @@ def write_ast_displaced_structures(
     cart_direction: NDArray[np.float64],
     amplitudes: NDArray[np.float64],
     filepaths: str | Path | list[str] | list[Path],
-    file_format: str,
+    file_format: str = "auto",
     overwrite: bool = False,
 ) -> None:
     """Write displaced structures with a single atom displaced along a direction.
@@ -180,7 +181,8 @@ def write_ast_displaced_structures(
         (Å) Array with shape (M,).
     filepaths
     file_format
-        Supports ``"poscar"`` (see :ref:`Supported formats`). Not case sensitive.
+        Supports ``"poscar"`` and ``"auto"`` (see :ref:`Supported formats`). Not case
+        sensitive.
     overwrite
         Overwrite the file if it exists.
     """
diff --git a/test/tests/test_outcar.py b/test/tests/test_outcar.py
index ef1be0c..99c0e9c 100644
--- a/test/tests/test_outcar.py
+++ b/test/tests/test_outcar.py
@@ -89,6 +89,6 @@ def test_read_trajectory_from_outcar(
     path_fixture: Path, trajectory_length: int, last_position: NDArray[np.float64]
 ) -> None:
     """Test read_trajectory for outcar (normal)."""
-    trajectory = generic_io.read_trajectory(path_fixture, file_format="outcar")
+    trajectory = generic_io.read_trajectory(path_fixture)
     assert len(trajectory) == trajectory_length
     assert np.allclose(last_position, trajectory[-1][-1])
diff --git a/test/tests/torch/test_dataset.py b/test/tests/torch/test_dataset.py
index 42961a8..18f67e2 100644
--- a/test/tests/torch/test_dataset.py
+++ b/test/tests/torch/test_dataset.py
@@ -27,7 +27,8 @@
     ],
 )
 def test_load_polarizability_dataset(
-    filepaths: str | list[str], file_format: str
+    filepaths: str | list[str],
+    file_format: str,
 ) -> None:
     """Test of generic load_polarizability_dataset (normal)."""
     dataset = generic_io.read_polarizability_dataset(filepaths, file_format)