diff --git a/crates/aaf/src/roundtrip_tests.rs b/crates/aaf/src/roundtrip_tests.rs
index 5ae2a6d8f..a949eef84 100644
--- a/crates/aaf/src/roundtrip_tests.rs
+++ b/crates/aaf/src/roundtrip_tests.rs
@@ -2,7 +2,7 @@
 
 use craft_timeline::time::{FrameRate, Tick};
 
-use crate::model::{CItem, CKind, CMarker, Document, Gain};
+use crate::model::{CItem, CKind, CMarker, Composition, Document, Gain};
 use crate::testdoc::{IDENTITY, pcm_ramp, picture_and_sound, sound_only};
 use crate::{Error, Report, aaf, omf};
 
@@ -90,9 +90,11 @@ fn aaf_round_trips_the_edit_at_every_rate() {
             let sound = back.sources.iter().find(|s| s.kind == CKind::Sound).unwrap();
             assert_eq!((sound.channels, sound.file_channels, sound.sample_rate, sound.bits), (2, 2, 48_000, 24), "{rate}");
             assert_eq!(sound.key, picture.key, "one media item");
-            // what was read, written again, reads as the same edit (mob ids, and so source keys, are new)
+            // what was read, written again, reads as the same edit (mob ids, and so source keys, are new;
+            // a writer generates fresh format identifiers and never preserves `aaf_mob_id`)
             let again = read_clean(&aaf_bytes(&back, small), true);
-            assert_eq!((shape(&again), &again.compositions), (shape(&back), &back.compositions), "{rate}");
+            let no_ids = |c: &[Composition]| c.iter().cloned().map(|mut c| { c.aaf_mob_id = None; c }).collect::<Vec<_>>();
+            assert_eq!((shape(&again), no_ids(&again.compositions)), (shape(&back), no_ids(&back.compositions)), "{rate}");
         }
     }
 }