diff --git a/crates/frontend-promql/src/promql.rs b/crates/frontend-promql/src/promql.rs
index 2d1cfa9f..bfea12b9 100644
--- a/crates/frontend-promql/src/promql.rs
+++ b/crates/frontend-promql/src/promql.rs
@@ -388,7 +388,14 @@ fn walk_binary(bin: &BinaryExpr) -> Result<L2> {
         let (kind, labels) = match &m.matching {
             Some(LabelModifier::Include(ls)) => (VectorMatchKind::On, ls.labels.clone()),
             Some(LabelModifier::Exclude(ls)) => (VectorMatchKind::Ignoring, ls.labels.clone()),
-            None => (VectorMatchKind::On, vec![]),
+            // No explicit `on(…)`/`ignoring(…)` — the parser attaches a default
+            // modifier to every set op (`and`/`or`/`unless`). The default is
+            // "match on all shared labels", which is exactly `ignoring([])`
+            // (ignore no labels). Representing it as `Ignoring([])` — not
+            // `On([])` — keeps it distinct from an explicit `on()` (match on the
+            // empty label set) while making it correctly equal to an explicit
+            // `ignoring()` (issue #68).
+            None => (VectorMatchKind::Ignoring, vec![]),
         };
         let grouping = match &m.card {
             VectorMatchCardinality::ManyToOne(ls) => Some(VectorGrouping {
diff --git a/crates/frontend-promql/tests/promql_equivalence.rs b/crates/frontend-promql/tests/promql_equivalence.rs
index b097e084..1dc2255f 100644
--- a/crates/frontend-promql/tests/promql_equivalence.rs
+++ b/crates/frontend-promql/tests/promql_equivalence.rs
@@ -141,6 +141,20 @@ fn rate_and_irate_share_the_same_intent() {
     assert_equiv(&["rate(m[5m])", "irate(m[5m])"]);
 }
 
+#[test]
+fn set_op_default_match_is_ignoring_empty_not_on_empty() {
+    // Issue #68. A set op's *default* matching ("match on all shared labels") is
+    // `ignoring([])`, NOT `on([])`. So:
+    //   - default `a and b` must stay DISTINCT from explicit `a and on() b`
+    //     (which matches on the empty label set), and
+    //   - default `a and b` must EQUAL explicit `a and ignoring() b`
+    //     (ignore no labels ⇒ match on all shared labels).
+    assert_distinct("a and b", "a and on() b");
+    assert_distinct("a or b", "a or on() b");
+    assert_equiv(&["a and b", "a and ignoring() b"]);
+    assert_equiv(&["a unless b", "a unless ignoring() b"]);
+}
+
 // ─────────────────────────────────────────────────────────────────────────────
 // 4. Distinct semantics we cannot faithfully represent are REJECTED, not
 //    silently merged into a wrong intent. (Each previously mislowered.)