diff --git a/interpreter/binary/decode.ml b/interpreter/binary/decode.ml
index 433b092ed..c4b0947bd 100644
--- a/interpreter/binary/decode.ml
+++ b/interpreter/binary/decode.ml
@@ -296,7 +296,7 @@ let globaltype s =
 
 let memorytype s =
   let at, lim = limits u64 s in
-  MemoryT (at, lim)
+  MemoryT (at, lim, PageT 0x10000) (* TODO(custom-page-sizes) *)
 
 let tabletype s =
   let t = reftype s in
diff --git a/interpreter/binary/encode.ml b/interpreter/binary/encode.ml
index 884592772..651bee528 100644
--- a/interpreter/binary/encode.ml
+++ b/interpreter/binary/encode.ml
@@ -194,7 +194,7 @@ struct
     | GlobalT (mut, t) -> valtype t; mutability mut
 
   let memorytype = function
-    | MemoryT (at, lim) -> limits at lim
+    | MemoryT (at, lim, pt) -> limits at lim (* TODO(custom-page-sizes) *)
 
   let tabletype = function
     | TableT (at, lim, t) -> reftype t; limits at lim
diff --git a/interpreter/host/spectest.ml b/interpreter/host/spectest.ml
index 03ad95424..82ee4be45 100644
--- a/interpreter/host/spectest.ml
+++ b/interpreter/host/spectest.ml
@@ -28,7 +28,7 @@ let table64 =
   ExternTable (Table.alloc tt (NullRef FuncHT))
 
 let memory =
-  let mt = MemoryT (I32AT, {min = 1L; max = Some 2L}) in
+  let mt = MemoryT (I32AT, {min = 1L; max = Some 2L}, PageT 0x10000) in
   ExternMemory (Memory.alloc mt)
 
 let func f ts1 ts2 =
diff --git a/interpreter/runtime/memory.ml b/interpreter/runtime/memory.ml
index 7914ebecc..9e73d6d2a 100644
--- a/interpreter/runtime/memory.ml
+++ b/interpreter/runtime/memory.ml
@@ -29,34 +29,37 @@ let valid_size at i =
   | I32AT -> I64.le_u i 0xffffL
   | I64AT -> true
 
-let create n =
+let create n (PageT ps) =
   try
-    let size = Int64.(mul n page_size) in
+    let size = Int64.(mul n (Int64.of_int ps)) in
     let mem = Array1_64.create Int8_unsigned C_layout size in
     Array1.fill mem 0;
     mem
   with Out_of_memory -> raise OutOfMemory
 
-let alloc (MemoryT (at, lim) as ty) =
+let alloc (MemoryT (at, lim, pt) as ty) =
   assert Free.((memorytype ty).types = Set.empty);
   if not (valid_size at lim.min) then raise SizeOverflow;
   if not (valid_limits lim) then raise Type;
-  {ty; content = create lim.min}
+  {ty; content = create lim.min pt}
 
 let bound mem =
   Array1_64.dim mem.content
 
+let pagesize mem =
+  let MemoryT (_, _, PageT x) = mem.ty in Int64.of_int x
+
 let size mem =
-  Int64.(div (bound mem) page_size)
+  Int64.(div (bound mem) (pagesize mem))
 
 let type_of mem =
   mem.ty
 
 let addrtype_of mem =
-  let MemoryT (at, _) = type_of mem in at
+  let MemoryT (at, _, _) = type_of mem in at
 
 let grow mem delta =
-  let MemoryT (at, lim) = mem.ty in
+  let MemoryT (at, lim, pt) = mem.ty in
   assert (lim.min = size mem);
   let old_size = lim.min in
   let new_size = Int64.add old_size delta in
@@ -64,10 +67,10 @@ let grow mem delta =
   let lim' = {lim with min = new_size} in
   if not (valid_size at new_size) then raise SizeOverflow else
   if not (valid_limits lim') then raise SizeLimit else
-  let after = create new_size in
+  let after = create new_size pt in
   let dim = Array1_64.dim mem.content in
   Array1.blit (Array1_64.sub mem.content 0L dim) (Array1_64.sub after 0L dim);
-  mem.ty <- MemoryT (at, lim');
+  mem.ty <- MemoryT (at, lim', pt);
   mem.content <- after
 
 let load_byte mem a =
diff --git a/interpreter/syntax/free.ml b/interpreter/syntax/free.ml
index a5c9f9b3c..d2eb30789 100644
--- a/interpreter/syntax/free.ml
+++ b/interpreter/syntax/free.ml
@@ -118,7 +118,7 @@ and deftype = function
 
 let tagtype (TagT ut) = typeuse ut
 let globaltype (GlobalT (_mut, t)) = valtype t
-let memorytype (MemoryT (_at, _lim)) = empty
+let memorytype (MemoryT (_at, _pt, _lim)) = empty
 let tabletype (TableT (_at, _lim, t)) = reftype t
 
 let externtype = function
diff --git a/interpreter/syntax/types.ml b/interpreter/syntax/types.ml
index fda781359..bd8066a87 100644
--- a/interpreter/syntax/types.ml
+++ b/interpreter/syntax/types.ml
@@ -13,6 +13,7 @@ type limits = {min : int64; max : int64 option}
 type typeuse = Idx of typeidx | Rec of int32 | Def of deftype
 
 and addrtype = I32AT | I64AT
+and pagetype = PageT of int
 and numtype = I32T | I64T | F32T | F64T
 and vectype = V128T
 and heaptype =
@@ -39,7 +40,7 @@ and deftype = DefT of rectype * int32
 
 type tagtype = TagT of typeuse
 type globaltype = GlobalT of mut * valtype
-type memorytype = MemoryT of addrtype * limits
+type memorytype = MemoryT of addrtype * limits * pagetype
 type tabletype = TableT of addrtype * limits * reftype
 type localtype = LocalT of init * valtype
 type externtype =
@@ -219,7 +220,7 @@ let subst_globaltype s = function
   | GlobalT (mut, t) ->  GlobalT (mut, subst_valtype s t)
 
 let subst_memorytype s = function
-  | MemoryT (at, lim) -> MemoryT (subst_addrtype s at, lim)
+  | MemoryT (at, pt, lim) -> MemoryT (subst_addrtype s at, pt, lim)
 
 let subst_tabletype s = function
   | TableT (at, lim, t) -> TableT (subst_addrtype s at, lim, subst_reftype s t)
@@ -322,6 +323,9 @@ let string_of_numtype = function
 let string_of_addrtype at =
   string_of_numtype (numtype_of_addrtype at)
 
+let string_of_pagetype = function
+  | PageT x -> string_of_int x
+
 let string_of_vectype = function
   | V128T -> "v128"
 
@@ -408,7 +412,7 @@ let string_of_globaltype = function
   | GlobalT (mut, t) -> string_of_mut (string_of_valtype t) mut
 
 let string_of_memorytype = function
-  | MemoryT (at, lim) -> string_of_addrtype at ^ " " ^ string_of_limits lim
+  | MemoryT (at, lim, pt) -> string_of_addrtype at ^ " " ^ string_of_limits lim ^ " " ^ string_of_pagetype pt
 
 let string_of_tabletype = function
   | TableT (at, lim, t) ->
diff --git a/interpreter/text/arrange.ml b/interpreter/text/arrange.ml
index 1ab2c222d..d9c987015 100644
--- a/interpreter/text/arrange.ml
+++ b/interpreter/text/arrange.ml
@@ -118,8 +118,8 @@ let tagtype (TagT ut) =
 let globaltype (GlobalT (mut, t)) =
   [mutability (atom string_of_valtype t) mut]
 
-let memorytype (MemoryT (at, lim)) =
-  [Atom (addrtype at ^ " " ^ limits nat64 lim)]
+let memorytype (MemoryT (at, lim, pt)) =
+  [Atom (addrtype at ^ " " ^ limits nat64 lim)] (* TODO(custom-page-sizes) *)
 
 let tabletype (TableT (at, lim, t)) =
   [Atom (addrtype at ^ " " ^ limits nat64 lim); atom reftype t]
diff --git a/interpreter/text/parser.mly b/interpreter/text/parser.mly
index 194590270..0ee8020c3 100644
--- a/interpreter/text/parser.mly
+++ b/interpreter/text/parser.mly
@@ -466,7 +466,7 @@ tabletype :
   | addrtype limits reftype { fun c -> TableT ($1, $2, $3 c) }
 
 memorytype :
-  | addrtype limits { fun c -> MemoryT ($1, $2) }
+  | addrtype limits { fun c -> MemoryT ($1, $2, PageT 0x10000) }
 
 limits :
   | NAT { {min = nat64 $1 $loc($1); max = None} }
@@ -1130,7 +1130,7 @@ memory_fields :
     { fun c x loc ->
       let size = Int64.(div (add (of_int (String.length $4)) 65535L) 65536L) in
       let offset = [at_const $1 (0L @@ loc) @@ loc] @@ loc in
-      [Memory (MemoryT ($1, {min = size; max = Some size})) @@ loc],
+      [Memory (MemoryT ($1, {min = size; max = Some size}, PageT 0x10000)) @@ loc],
       [Data ($4, Active (x, offset) @@ loc) @@ loc],
       [], [] }
 
diff --git a/interpreter/valid/match.ml b/interpreter/valid/match.ml
index e82706478..cd4083fad 100644
--- a/interpreter/valid/match.ml
+++ b/interpreter/valid/match.ml
@@ -165,8 +165,8 @@ let match_globaltype c (GlobalT (mut1, t1)) (GlobalT (mut2, t2)) =
   | Cons -> true
   | Var -> match_valtype c t2 t1
 
-let match_memorytype c (MemoryT (at1, lim1)) (MemoryT (at2, lim2)) =
-  at1 = at2 && match_limits c lim1 lim2
+let match_memorytype c (MemoryT (at1, lim1, pt1)) (MemoryT (at2, lim2, pt2)) =
+  at1 = at2 && pt1 = pt2 && match_limits c lim1 lim2
 
 let match_tabletype c (TableT (at1, lim1, t1)) (TableT (at2, lim2, t2)) =
   at1 = at2 && match_limits c lim1 lim2 &&
diff --git a/interpreter/valid/valid.ml b/interpreter/valid/valid.ml
index cb41335b8..c22a1d0cf 100644
--- a/interpreter/valid/valid.ml
+++ b/interpreter/valid/valid.ml
@@ -104,6 +104,9 @@ let check_limits {min; max} range at msg =
     require (I64.le_u min max) at
       "size minimum must not be greater than maximum"
 
+let check_pagetype (PageT ps) at =
+  require (ps = 0x10000 || ps = 1) at "page size must be 1 or 64KiB"
+
 let check_numtype (c : context) (t : numtype) at =
   ()
 
@@ -196,13 +199,14 @@ let check_globaltype (c : context) (gt : globaltype) at =
   check_valtype c t at
 
 let check_memorytype (c : context) (mt : memorytype) at =
-  let MemoryT (at_, lim) = mt in
+  let MemoryT (at_, lim, pt) = mt in
   let sz, s =
     match at_ with
     | I32AT -> 0x1_0000L, "2^16 pages (4 GiB) for i32"
     | I64AT -> 0x1_0000_0000_0000L, "2^48 pages (256 TiB) for i64"
   in
-  check_limits lim sz at ("memory size must be at most " ^ s)
+  check_limits lim sz at ("memory size must be at most " ^ s);
+  check_pagetype pt at
 
 let check_tabletype (c : context) (tt : tabletype) at =
   let TableT (at_, lim, t) = tt in
@@ -384,7 +388,7 @@ let check_memop (c : context) (memop : ('t, 's) memop) ty_size get_sz at =
   in
   require (1 lsl memop.align >= 1 && 1 lsl memop.align <= size) at
     "alignment must not be larger than natural";
-  let MemoryT (at_, _lim) = memory c (0l @@ at) in
+  let MemoryT (at_, _lim, _pt) = memory c (0l @@ at) in
   if at_ = I32AT then
     require (I64.lt_u memop.offset 0x1_0000_0000L) at
       "offset out of range";
@@ -648,60 +652,60 @@ let rec check_instr (c : context) (e : instr) (s : infer_resulttype) : infer_ins
     [] --> [], []
 
   | Load (x, memop) ->
-    let MemoryT (at, _lim) = memory c x in
+    let MemoryT (at, _lim, _pt) = memory c x in
     let t = check_memop c memop num_size (Lib.Option.map fst) e.at in
     [NumT (numtype_of_addrtype at)] --> [NumT t], []
 
   | Store (x, memop) ->
-    let MemoryT (at, _lim) = memory c x in
+    let MemoryT (at, _lim, _pt) = memory c x in
     let t = check_memop c memop num_size (fun sz -> sz) e.at in
     [NumT (numtype_of_addrtype at); NumT t] --> [], []
 
   | VecLoad (x, memop) ->
-    let MemoryT (at, _lim) = memory c x in
+    let MemoryT (at, _lim, _pt) = memory c x in
     let t = check_memop c memop vec_size (Lib.Option.map fst) e.at in
     [NumT (numtype_of_addrtype at)] --> [VecT t], []
 
   | VecStore (x, memop) ->
-    let MemoryT (at, _lim) = memory c x in
+    let MemoryT (at, _lim, _pt) = memory c x in
     let t = check_memop c memop vec_size (fun _ -> None) e.at in
     [NumT (numtype_of_addrtype at); VecT t] --> [], []
 
   | VecLoadLane (x, memop, i) ->
-    let MemoryT (at, _lim) = memory c x in
+    let MemoryT (at, _lim, _pt) = memory c x in
     let t = check_memop c memop vec_size (fun sz -> Some sz) e.at in
     require (I8.to_int_u i < vec_size t / Pack.packed_size memop.pack) e.at
       "invalid lane index";
     [NumT (numtype_of_addrtype at); VecT t] -->  [VecT t], []
 
   | VecStoreLane (x, memop, i) ->
-    let MemoryT (at, _lim) = memory c x in
+    let MemoryT (at, _lim, _pt) = memory c x in
     let t = check_memop c memop vec_size (fun sz -> Some sz) e.at in
     require (I8.to_int_u i < vec_size t / Pack.packed_size memop.pack) e.at
       "invalid lane index";
     [NumT (numtype_of_addrtype at); VecT t] -->  [], []
 
   | MemorySize x ->
-    let MemoryT (at, _lim) = memory c x in
+    let MemoryT (at, _lim, _pt) = memory c x in
     [] --> [NumT (numtype_of_addrtype at)], []
 
   | MemoryGrow x ->
-    let MemoryT (at, _lim) = memory c x in
+    let MemoryT (at, _lim, _pt) = memory c x in
     [NumT (numtype_of_addrtype at)] --> [NumT (numtype_of_addrtype at)], []
 
   | MemoryFill x ->
-    let MemoryT (at, _lim) = memory c x in
+    let MemoryT (at, _lim, _pt) = memory c x in
     [NumT (numtype_of_addrtype at); NumT I32T;
       NumT (numtype_of_addrtype at)] --> [], []
 
   | MemoryCopy (x, y)->
-    let MemoryT (at1, _lib1) = memory c x in
-    let MemoryT (at2, _lib2) = memory c y in
+    let MemoryT (at1, _pt, _lib1) = memory c x in
+    let MemoryT (at2, _pt, _lib2) = memory c y in
     [NumT (numtype_of_addrtype at1); NumT (numtype_of_addrtype at2);
       NumT (numtype_of_addrtype (min at1 at2))] --> [], []
 
   | MemoryInit (x, y) ->
-    let MemoryT (at, _lib) = memory c x in
+    let MemoryT (at, _pt, _lib) = memory c x in
     let () = data c y in
     [NumT (numtype_of_addrtype at); NumT I32T; NumT I32T] --> [], []
 
@@ -1071,7 +1075,7 @@ let check_datamode (c : context) (mode : segmentmode) =
   match mode.it with
   | Passive -> ()
   | Active (x, offset) ->
-    let MemoryT (at, _) = memory c x in
+    let MemoryT (at, _, _) = memory c x in
     check_const c offset (NumT (numtype_of_addrtype at))
   | Declarative -> assert false