diff --git a/lib/ocaml/baseline/adler32.ml b/lib/ocaml/baseline/adler32.ml
new file mode 100644
index 00000000..aa802806
--- /dev/null
+++ b/lib/ocaml/baseline/adler32.ml
@@ -0,0 +1,13 @@
+let adler32 s =
+  let a = ref 1 in
+  let b = ref 0 in
+  let m = 65521 in
+  for i = 0 to String.length s - 1 do
+    a := (!a + Char.code s.[i]) mod m;
+    b := (!b + !a) mod m
+  done;
+  !b * 65536 + !a
+
+;;
+
+adler32 "Wikipedia"
diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 019e45a1..4262f1f0 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -1,5 +1,6 @@
 {
   "ackermann.ml": 125,
+  "adler32.ml": 300286872,
   "anagram_check.ml": 2,
   "anagrams.ml": 3,
   "atm.ml": 120,
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 0632af61..01e2f3a0 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,11 @@ _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 — adler32.ml baseline (Adler-32 checksum of
+  "Wikipedia" = 300286872 = 0x11E60398). Two running sums modulo
+  65521; final checksum is `b * 65536 + a`. Used by zlib for stream
+  integrity. Tests for-loop accumulating two refs, modular
+  arithmetic, and Char.code on s.[i]. 96 baseline programs total.
 - 2026-05-09 Phase 5.1 — gray_code.ml baseline (4-bit binary
   reflected Gray code, sum 120 + length 16 = 136). Single-formula
   generation: `gray[i] = i lxor (i lsr 1)`. Outputs a permutation of