diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 862e864c..95229766 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -74,6 +74,7 @@
   "option_match.ml": 5,
   "palindrome.ml": 4,
   "paren_depth.ml": 7,
+  "partition.ml": 3025,
   "pancake_sort.ml": 910,
   "pascal.ml": 252,
   "peano.ml": 30,
diff --git a/lib/ocaml/baseline/partition.ml b/lib/ocaml/baseline/partition.ml
new file mode 100644
index 00000000..b1cf0bf6
--- /dev/null
+++ b/lib/ocaml/baseline/partition.ml
@@ -0,0 +1,13 @@
+let partition pred xs =
+  let yes = ref [] in
+  let no = ref [] in
+  List.iter (fun x ->
+    if pred x then yes := x :: !yes
+    else no := x :: !no
+  ) xs;
+  (List.rev !yes, List.rev !no)
+
+;;
+
+let (evens, odds) = partition (fun x -> x mod 2 = 0) [1;2;3;4;5;6;7;8;9;10] in
+List.fold_left (+) 0 evens * 100 + List.fold_left (+) 0 odds
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 56749bed..5b89f459 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,13 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-09 Phase 5.1 — partition.ml baseline (stable partition by
+  predicate, 30*100 + 25 = 3025). Two ref lists accumulating in
+  reverse, then List.rev'd — preserves original order. Test:
+  `partition (fun x -> x mod 2 = 0) [1..10]` → ([2;4;6;8;10],
+  [1;3;5;7;9]) → 30*100 + 25 = 3025. Tests higher-order predicate
+  + tuple return + iter-98 let-tuple destructuring. 108 baseline
+  programs total.
 - 2026-05-09 Phase 5.1 — is_prime.ml baseline (count primes ≤ 100 =
   25). Trial division up to √n with early-exit via bool ref. Loop
   2..n calling is_prime, accumulate count. Returns 25 (the canonical