diff --git a/clinica/utils/longitudinal.py b/clinica/utils/longitudinal.py
index 082caf44a..4f0331845 100644
--- a/clinica/utils/longitudinal.py
+++ b/clinica/utils/longitudinal.py
@@ -26,16 +26,16 @@ def get_long_id(session_ids: List[str]) -> str:
     >>> get_long_id(['ses-M000'])
     'long-M000'
     >>> get_long_id(['ses-M000', 'ses-M018', 'ses-M036'])
-    'long-M000M018M036'
+    'long-M000.M018.M036'
     >>> get_long_id(['ses-M018', 'ses-M036', 'ses-M000'])
-    'long-M000M018M036'
+    'long-M000.M018.M036'
     """
     if not all([session_id.startswith("ses-") for session_id in session_ids]):
         raise ValueError(
             "Expected a list of session IDs of the form ses-XXX, "
             f"but received {session_ids} instead."
         )
-    return "long-" + "".join(
+    return "long-" + ".".join(
         [session_id.lstrip("ses-") for session_id in sorted(session_ids)]
     )
 
@@ -62,7 +62,7 @@ def get_participants_long_id(
     --------
     >>> from clinica.utils.longitudinal import get_participants_long_id
     >>> get_participants_long_id(['sub-CLNC01', 'sub-CLNC01', 'sub-CLNC02'], ['ses-M000', 'ses-M018', 'ses-M000'])
-    ['long-M000M018', 'long-M000M018', 'long-M000']
+    ['long-M000.M018', 'long-M000.M018', 'long-M000']
     """
     from .participant import get_unique_subjects
 
diff --git a/test/unittests/utils/test_longitudinal.py b/test/unittests/utils/test_longitudinal.py
index 5d39e9706..0d6aff9e5 100644
--- a/test/unittests/utils/test_longitudinal.py
+++ b/test/unittests/utils/test_longitudinal.py
@@ -5,9 +5,9 @@
     "session_ids,expected",
     [
         (["ses-M000"], "long-M000"),
-        (["ses-M000", "ses-M018", "ses-M036"], "long-M000M018M036"),
-        (["ses-M018", "ses-M036", "ses-M000"], "long-M000M018M036"),
-        (["ses-foo", "ses-bar", "ses-baz", "ses-foobar"], "long-barbazfoofoobar"),
+        (["ses-M000", "ses-M018", "ses-M036"], "long-M000.M018.M036"),
+        (["ses-M018", "ses-M036", "ses-M000"], "long-M000.M018.M036"),
+        (["ses-foo", "ses-bar", "ses-baz", "ses-foobar"], "long-bar.baz.foo.foobar"),
     ],
 )
 def test_get_long_id(session_ids, expected):
@@ -40,7 +40,7 @@ def test_get_long_id_errors(session_ids):
         (
             ["sub-CLNC01", "sub-CLNC01", "sub-CLNC02"],
             ["ses-M000", "ses-M018", "ses-M000"],
-            ["long-M000M018", "long-M000M018", "long-M000"],
+            ["long-M000.M018", "long-M000.M018", "long-M000"],
         )
     ],
 )
@@ -64,8 +64,8 @@ def test_save_long_id(tmp_path):
 
     save_long_id(["ses-M000", "ses-M018", "ses-M036"], output_dir)
 
-    assert (output_dir / "long-M000M018M036_sessions.tsv").exists()
-    saved_ids = (output_dir / "long-M000M018M036_sessions.tsv").read_text()
+    assert (output_dir / "long-M000.M018.M036_sessions.tsv").exists()
+    saved_ids = (output_dir / "long-M000.M018.M036_sessions.tsv").read_text()
     assert saved_ids == "session_id\nses-M000\nses-M018\nses-M036\n"