diff --git a/lib/ocaml/baseline/balance.ml b/lib/ocaml/baseline/balance.ml
new file mode 100644
index 00000000..5afcafa7
--- /dev/null
+++ b/lib/ocaml/baseline/balance.ml
@@ -0,0 +1,25 @@
+let is_balanced s =
+  let stack = Stack.create () in
+  let n = String.length s in
+  let ok = ref true in
+  let i = ref 0 in
+  while !i < n && !ok do
+    let c = s.[!i] in
+    (if c = '(' || c = '[' || c = '{' then Stack.push c stack
+     else if c = ')' then
+       (if Stack.is_empty stack || Stack.pop stack <> '(' then ok := false)
+     else if c = ']' then
+       (if Stack.is_empty stack || Stack.pop stack <> '[' then ok := false)
+     else if c = '}' then
+       (if Stack.is_empty stack || Stack.pop stack <> '{' then ok := false));
+    i := !i + 1
+  done;
+  !ok && Stack.is_empty stack
+
+;;
+
+(if is_balanced "({[abc]d}e)" then 1 else 0) +
+(if is_balanced "(a]" then 1 else 0) +
+(if is_balanced "{[}]" then 1 else 0) +
+(if is_balanced "(())" then 1 else 0) +
+(if is_balanced "" then 1 else 0)
diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 7a081af7..47733ffd 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -1,6 +1,7 @@
 {
   "anagrams.ml": 3,
   "bag.ml": 3,
+  "balance.ml": 3,
   "bfs.ml": 6,
   "btree.ml": 39,
   "brainfuck.ml": 75,
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 3b0ad6db..262543f8 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 — balance.ml baseline (paren/bracket/brace
+  balance using Stack). is_balanced walks a string; on opener push,
+  on closer check stack non-empty + top matches expected opener (else
+  fail). Returns ok && is_empty stack at end. 5 test cases:
+  "({[abc]d}e)" ✓, "(a]" ✗, "{[}]" ✗ (mismatched closers), "(())" ✓,
+  "" ✓ → 3 balanced. Exercises Stack.create / push / pop / is_empty /
+  s.[!i] / while + bool ref short-circuit. 31 baseline programs total.
 - 2026-05-09 Phase 5.1 — safe_div.ml baseline + Result.equal /
   compare / iter_error (+3 tests, 592 total). safe_div divides only
   if divisor non-zero, returns `Error "..."` otherwise. sum_safe folds