diff --git a/extra_data/reader.py b/extra_data/reader.py
index 2ebd07b9..b31f0a78 100644
--- a/extra_data/reader.py
+++ b/extra_data/reader.py
@@ -645,7 +645,7 @@ def _load_aliases_from_file(self, aliases_path):
             with open(aliases_path, 'r') as f:
                 data = json.load(f)
 
-        elif aliases_path.suffix == '.yaml':
+        elif aliases_path.suffix in ['.yaml', '.yml']:
             import yaml
 
             with open(aliases_path, 'r') as f:
@@ -1617,6 +1617,13 @@ def __getitem__(self, aliased_item):
 
         raise TypeError('expected alias or (source alias, key) tuple')
 
+    def __contains__(self, aliased_item):
+        try:
+            self[aliased_item]
+            return True
+        except KeyError:
+            return False
+
     def _resolve_aliased_selection(self, selection):
         if isinstance(selection, dict):
             res = {self._resolve_source_alias(alias): keys
@@ -1786,10 +1793,12 @@ def RunDirectory(
 # any code was already using it.
 RunHandler = RunDirectory
 
+DEFAULT_ALIASES_FILE = "{}/usr/extra-data-aliases.yml"
 
 def open_run(
         proposal, run, data='raw', include='*', file_filter=locality.lc_any, *,
-        inc_suspect_trains=True, parallelize=True, _use_voview=True,
+        inc_suspect_trains=True, parallelize=True, aliases=DEFAULT_ALIASES_FILE,
+        _use_voview=True,
 ):
     """Access EuXFEL data on the Maxwell cluster by proposal and run number.
 
@@ -1824,6 +1833,13 @@ def open_run(
         Enable or disable opening files in parallel. Particularly useful if
         creating child processes is not allowed (e.g. in a daemonized
         :class:`multiprocessing.Process`).
+    aliases: str, Path
+        Path to an alias file for the run, see the documentation for
+        :meth:`DataCollection.with_aliases` for details. If the
+        argument is a string with a format argument like
+        ``{}/path/to/aliases.yml``, then the format argument will be replaced with
+        the proposal directory path. By default it looks for a file named
+        ``{}/usr/extra-data-aliases.yml``.
     """
     if data == 'all':
         common_args = dict(
@@ -1871,8 +1887,24 @@ def open_run(
         run = index(run)  # Allow integers, including numpy integers
     run = 'r' + str(run).zfill(4)
 
-    return RunDirectory(
+    dc = RunDirectory(
         osp.join(prop_dir, data, run), include=include, file_filter=file_filter,
         inc_suspect_trains=inc_suspect_trains, parallelize=parallelize,
         _use_voview=_use_voview,
     )
+
+    # Normalize string arguments to be an absolute Path
+    if isinstance(aliases, str):
+        aliases = Path(aliases.format(prop_dir))
+
+    # If we're using the default aliases file and it doesn't exist, ignore it
+    # without throwing any errors.
+    default_aliases = Path(DEFAULT_ALIASES_FILE.format(prop_dir))
+    if aliases == default_aliases and not default_aliases.is_file():
+        aliases = None
+
+    if aliases is not None:
+        dc = dc.with_aliases(aliases)
+        print(f"Loading {len(dc._aliases)} aliases from: {aliases}", flush=True, file=sys.stderr)
+
+    return dc
diff --git a/extra_data/tests/test_aliases.py b/extra_data/tests/test_aliases.py
index 750fb509..502334ea 100644
--- a/extra_data/tests/test_aliases.py
+++ b/extra_data/tests/test_aliases.py
@@ -23,6 +23,12 @@ def assert_equal_keydata(kd1, kd2):
     # Test whether source alias yields identical SourceData.
     assert run.alias['sa3-xgm'] is run['SA3_XTD10_XGM/XGM/DOOCS']
 
+    # Test __contains__()
+    assert "sa3-xgm" in run.alias
+    assert not "sa42-xgm" in run.alias
+    with pytest.raises(TypeError):
+        42 in run.alias
+
     # Test whether source alias plus literal key yields equal KeyData.
     assert_equal_keydata(
         run.alias['sa3-xgm', 'pulseEnergy.wavelengthUsed'],
diff --git a/extra_data/tests/test_reader_mockdata.py b/extra_data/tests/test_reader_mockdata.py
index 5f6de8b6..23d61ed3 100644
--- a/extra_data/tests/test_reader_mockdata.py
+++ b/extra_data/tests/test_reader_mockdata.py
@@ -1,6 +1,8 @@
 from datetime import datetime, timedelta, timezone
 from itertools import islice
 from multiprocessing import Process
+from pathlib import Path
+from textwrap import dedent
 from warnings import catch_warnings
 
 import h5py
@@ -20,6 +22,7 @@
     SourceNameError, PropertyNameError, DataCollection, open_run,
     MultiRunError
 )
+from extra_data.reader import DEFAULT_ALIASES_FILE
 
 def test_iterate_trains(mock_agipd_data, mock_control_data_with_empty_source):
     with H5File(mock_agipd_data) as f:
@@ -850,6 +853,36 @@ def test_open_run(mock_spb_raw_and_proc_run):
             open_run(proposal=2012, run=238, data='all')
             assert len(w) == 1
 
+        # Helper function to write an alias file at a specific path
+        def write_aliases(path):
+            aliases_path.parent.mkdir(parents=True, exist_ok=True)
+            aliases_path.write_text(dedent("""
+            xgm: SA1_XTD2_XGM/DOOCS/MAIN
+            """))
+
+        # To set the aliases, we should be able to use a string relative to the
+        # proposal directory.
+        aliases_path = Path(mock_data_root) / "SPB/201830/p002012/foo.yml"
+        write_aliases(aliases_path)
+        run = open_run(2012, 238, aliases="{}/foo.yml")
+        assert "xgm" in run.alias
+
+        # And a proper path
+        aliases_path = Path(mock_data_root) / "foo.yml"
+        write_aliases(aliases_path)
+        run = open_run(2012, 238, aliases=aliases_path)
+        assert "xgm" in run.alias
+
+        # And a plain string
+        run = open_run(2012, 238, aliases=str(aliases_path))
+        assert "xgm" in run.alias
+
+        # If the default file exists, it should be used automatically
+        aliases_path = Path(DEFAULT_ALIASES_FILE.format(mock_data_root + "/SPB/201830/p002012"))
+        write_aliases(aliases_path)
+        run = open_run(2012, 238)
+        assert "xgm" in run.alias
+
 def test_open_file(mock_sa3_control_data):
     f = H5File(mock_sa3_control_data)
     file_access = f.files[0]