diff --git a/crates/idml/src/export.rs b/crates/idml/src/export.rs
index 9d73562b9..610204d83 100644
--- a/crates/idml/src/export.rs
+++ b/crates/idml/src/export.rs
@@ -2055,7 +2055,8 @@ impl<'a> Ex<'a> {
                                     .attr("VerticalReferencePoint", names::anchor_rel_out(*y_relative, true))
                                     .attr("HorizontalAlignment", ["LeftAlign", "CenterAlign", "RightAlign"][(*ref_point % 3) as usize])
                                     .attr("VerticalAlignment", ["TopAlign", "CenterAlign", "BottomAlign"][(*ref_point / 3).min(2) as usize])
-                                    .attr("AnchorXoffset", num(*x_offset))
+                                    // IDML measures a left-aligned object's X offset leftwards.
+                                    .attr("AnchorXoffset", num(if *ref_point % 3 == 0 { 0.0 - *x_offset } else { *x_offset }))
                                     .attr("AnchorYoffset", num(*y_offset))
                                     .attr("PinPosition", bool_s(*keep_within_column)),
                             };
diff --git a/crates/idml/src/import.rs b/crates/idml/src/import.rs
index ddd51dc06..916f9cc66 100644
--- a/crates/idml/src/import.rs
+++ b/crates/idml/src/import.rs
@@ -1876,7 +1876,8 @@ impl<'r> Importer<'r> {
                                     designcraft_doc::AnchorPosition::Custom {
                                         x_relative: names::anchor_rel_in(s("HorizontalReferencePoint")),
                                         y_relative: names::anchor_rel_in(s("VerticalReferencePoint")),
-                                        x_offset: get("AnchorXoffset"),
+                                        // IDML measures a left-aligned object's X offset leftwards.
+                                        x_offset: if col == 0 { 0.0 - get("AnchorXoffset") } else { get("AnchorXoffset") },
                                         y_offset: get("AnchorYoffset"),
                                         object_point: names::anchor_in(s("AnchorPoint")),
                                         ref_point: row * 3 + col,
diff --git a/crates/idml/src/tests.rs b/crates/idml/src/tests.rs
index 3035d75d3..4a8642afb 100644
--- a/crates/idml/src/tests.rs
+++ b/crates/idml/src/tests.rs
@@ -822,13 +822,65 @@ fn anchored_objects_take_unset_settings_from_object_styles() {
     assert_eq!(
         positions,
         [
-            custom(R::TextFrame, R::Anchor, 5.0, -3.0, 8, true),
+            custom(R::TextFrame, R::Anchor, -5.0, -3.0, 8, true),
             custom(R::TextFrame, R::PageEdge, 0.0, 0.0, 0, true),
-            custom(R::PageMargin, R::Anchor, 2.0, 0.0, 8, false),
+            custom(R::PageMargin, R::Anchor, -2.0, 0.0, 8, false),
         ]
     );
 }
 
+/// IDML measures a left-aligned anchored object's X offset leftwards; the document model and
+/// composition measure every X offset rightwards.
+#[test]
+fn left_aligned_anchored_x_offsets_flip_at_the_idml_boundary() {
+    use designcraft_doc::AnchorPosition;
+    let rect = |id: &str, align: &str, x: &str| {
+        format!(
+            r#"<Rectangle Self="{id}" ItemTransform="1 0 0 1 0 0">
+              <Properties><PathGeometry><GeometryPathType PathOpen="false"><PathPointArray>
+                <PathPointType Anchor="0 0"/><PathPointType Anchor="0 10"/><PathPointType Anchor="10 10"/><PathPointType Anchor="10 0"/>
+              </PathPointArray></GeometryPathType></PathGeometry></Properties>
+              <AnchoredObjectSetting AnchoredPosition="Anchored" AnchorPoint="TopLeftAnchor" HorizontalReferencePoint="TextFrame"
+                HorizontalAlignment="{align}" VerticalAlignment="TopAlign" VerticalReferencePoint="PageEdge" AnchorXoffset="{x}" AnchorYoffset="0"/>
+            </Rectangle>"#
+        )
+    };
+    let story = format!(
+        r#"<idPkg:Story xmlns:idPkg="http://ns.adobe.com/AdobeInDesign/idml/1.0/packaging"><Story Self="s1">
+          <ParagraphStyleRange><CharacterStyleRange><Content>a</Content>{}{}{}</CharacterStyleRange></ParagraphStyleRange>
+        </Story></idPkg:Story>"#,
+        rect("left", "LeftAlign", "-108"),
+        rect("zero", "LeftAlign", "0"),
+        rect("center", "CenterAlign", "116"),
+    );
+    let d = import_idml(&fixture_with_story(&story)).unwrap();
+    let x_offsets = |d: &Document| -> Vec<f64> {
+        let s = d.stories.values().find(|s| !s.objects.is_empty()).unwrap();
+        s.objects.iter().map(|o| if let AnchorPosition::Custom { x_offset, .. } = o.position { x_offset } else { f64::NAN }).collect()
+    };
+    assert_eq!(x_offsets(&d), [108.0, 0.0, 116.0]);
+    // The left-aligned object lands 108 pt right of the reference point.
+    let sid = d.stories.iter().find(|(_, s)| !s.objects.is_empty()).map(|(id, _)| *id).unwrap();
+    let cs = designcraft_compose::compose_story(&d, sid, &Default::default());
+    let placed: Vec<_> = cs.frames.iter().flat_map(|f| &f.objects).collect();
+    let x = |i: usize| placed.iter().find(|o| o.index == i).unwrap().origin.x;
+    assert!((x(0) - x(1) - 108.0).abs() < 1e-6, "{} {}", x(0), x(1));
+    // Export writes the original IDML values back.
+    let bytes = export_idml(&d);
+    let mut zip = zip::ZipArchive::new(std::io::Cursor::new(bytes)).unwrap();
+    let mut stories = String::new();
+    for i in 0..zip.len() {
+        let mut f = zip.by_index(i).unwrap();
+        if f.name().starts_with("Stories/") {
+            std::io::Read::read_to_string(&mut f, &mut stories).unwrap();
+        }
+    }
+    for v in [r#"AnchorXoffset="-108""#, r#"AnchorXoffset="0""#, r#"AnchorXoffset="116""#] {
+        assert!(stories.contains(v), "{v} missing");
+    }
+    assert_eq!(x_offsets(&import_idml(&export_idml(&d)).unwrap()), [108.0, 0.0, 116.0]);
+}
+
 #[test]
 fn round_trips_frames_with_pasted_in_items() {
     let mut d = Document::new(&NewDocument::default());