diff --git a/src/proto_alpha/lib_protocol/operation_repr.ml b/src/proto_alpha/lib_protocol/operation_repr.ml
index dd9c7b34b21a5e2c09f6d12cd9275c721aa6e5db..c524f5951e0ed403b6d9638b3997a7a0a603ad09 100644
--- a/src/proto_alpha/lib_protocol/operation_repr.ml
+++ b/src/proto_alpha/lib_protocol/operation_repr.ml
@@ -804,12 +804,6 @@ module Encoding = struct
           inj = (fun () -> Tx_rollup_origination);
         }
 
-    let tx_rollup_batch_content =
-      (* The content of batches is a string, but stands for an immutable byte
-         sequence. JSON only allows unicode strings so we use the [bytes]
-         encoding which is in hexadecimal for JSON. *)
-      conv Bytes.of_string Bytes.to_string bytes
-
     let tx_rollup_submit_batch_case =
       MCase
         {
@@ -818,7 +812,7 @@ module Encoding = struct
           encoding =
             obj3
               (req "rollup" Tx_rollup_repr.encoding)
-              (req "content" tx_rollup_batch_content)
+              (req "content" (string' Hex))
               (opt "burn_limit" Tez_repr.encoding);
           select =
             (function
@@ -1126,9 +1120,6 @@ module Encoding = struct
             (fun (zk_rollup, update) -> Zk_rollup_update {zk_rollup; update});
         }
 
-    let string_to_bytes_encoding =
-      Data_encoding.conv Bytes.of_string Bytes.to_string Data_encoding.bytes
-
     let sc_rollup_originate_case =
       MCase
         {
@@ -1137,8 +1128,8 @@ module Encoding = struct
           encoding =
             obj4
               (req "pvm_kind" Sc_rollups.Kind.encoding)
-              (req "boot_sector" string_to_bytes_encoding)
-              (req "origination_proof" string_to_bytes_encoding)
+              (req "boot_sector" (string' Hex))
+              (req "origination_proof" (string' Hex))
               (req "parameters_ty" Script_repr.lazy_expr_encoding);
           select =
             (function
@@ -1173,7 +1164,7 @@ module Encoding = struct
         {
           tag = sc_rollup_operation_add_message_tag;
           name = "sc_rollup_add_messages";
-          encoding = obj1 (req "message" (list string));
+          encoding = obj1 (req "message" (list (string' Hex)));
           select =
             (function
             | Manager (Sc_rollup_add_messages _ as op) -> Some op | _ -> None);
@@ -1270,7 +1261,7 @@ module Encoding = struct
               (req
                  "cemented_commitment"
                  Sc_rollup_commitment_repr.Hash.encoding)
-              (req "output_proof" Data_encoding.string);
+              (req "output_proof" (string' Hex));
           select =
             (function
             | Manager (Sc_rollup_execute_outbox_message _ as op) -> Some op
@@ -1589,7 +1580,7 @@ module Encoding = struct
       {
         tag = 17;
         name = "failing_noop";
-        encoding = obj1 (req "arbitrary" Data_encoding.string);
+        encoding = obj1 (req "arbitrary" (string' Hex));
         select =
           (function Contents (Failing_noop _ as op) -> Some op | _ -> None);
         proj = (function Failing_noop message -> message);
@@ -1948,7 +1939,7 @@ let () =
         ppf
         "An operation contents list has an unexpected shape: %s"
         s)
-    Data_encoding.(obj1 (req "message" string))
+    Data_encoding.(obj1 (req "message" (string' Hex)))
     (function Contents_list_error s -> Some s | _ -> None)
     (fun s -> Contents_list_error s)
 
diff --git a/src/proto_alpha/lib_protocol/sc_rollup_PVM_sig.ml b/src/proto_alpha/lib_protocol/sc_rollup_PVM_sig.ml
index c4a4d6ca4d7ab37c7d7618b1b44a6ecf0ef6bd27..8b8b3a623219222565f0ba3f22f79f9aa53c0fc5 100644
--- a/src/proto_alpha/lib_protocol/sc_rollup_PVM_sig.ml
+++ b/src/proto_alpha/lib_protocol/sc_rollup_PVM_sig.ml
@@ -102,7 +102,7 @@ let inbox_message_encoding =
     (obj3
        (req "inbox_level" Raw_level_repr.encoding)
        (req "message_counter" n)
-       (req "payload" string))
+       (req "payload" (string' Hex)))
 
 let reveal_data_encoding =
   let open Data_encoding in
@@ -114,9 +114,11 @@ let reveal_data_encoding =
          (req "reveal_data_kind" (constant "raw_data"))
          (req
             "raw_data"
-            (check_size Constants_repr.sc_rollup_message_size_limit bytes)))
-      (function Raw_data m -> Some ((), Bytes.of_string m) | _ -> None)
-      (fun ((), m) -> Raw_data (Bytes.to_string m))
+            (check_size
+               Constants_repr.sc_rollup_message_size_limit
+               (string' Hex))))
+      (function Raw_data m -> Some ((), m) | _ -> None)
+      (fun ((), m) -> Raw_data m)
   and case_metadata =
     case
       ~title:"metadata"
diff --git a/src/proto_alpha/lib_protocol/sc_rollup_inbox_message_repr.ml b/src/proto_alpha/lib_protocol/sc_rollup_inbox_message_repr.ml
index 62effb1292f070d25ecd8ef88aa0e0bbc40d29d8..54e2c73c2b1d23e4643e2596c9a0a7459ed558a6 100644
--- a/src/proto_alpha/lib_protocol/sc_rollup_inbox_message_repr.ml
+++ b/src/proto_alpha/lib_protocol/sc_rollup_inbox_message_repr.ml
@@ -98,9 +98,6 @@ let internal_inbox_message_encoding =
         (fun () -> End_of_level);
     ]
 
-(* TODO: https://gitlab.com/tezos/tezos/-/issues/4027
-   We should change the payload of [External] from [bytes] to [string]. *)
-
 type t = Internal of internal_inbox_message | External of string
 
 let encoding =
@@ -120,7 +117,7 @@ let encoding =
          case
            (Tag 1)
            ~title:"External"
-           Variable.string
+           Variable.(string' Hex)
            (function External msg -> Some msg | Internal _ -> None)
            (fun msg -> External msg);
        ])
diff --git a/src/proto_alpha/lib_protocol/sc_rollup_inbox_repr.ml b/src/proto_alpha/lib_protocol/sc_rollup_inbox_repr.ml
index f1084903669303f80f1351da2eea2d774e0b8cae..855088a64605fb11098acfddae57bb0f31671c5a 100644
--- a/src/proto_alpha/lib_protocol/sc_rollup_inbox_repr.ml
+++ b/src/proto_alpha/lib_protocol/sc_rollup_inbox_repr.ml
@@ -352,9 +352,9 @@ let key_of_message ix =
 
 let number_of_messages_key = ["number_of_messages"]
 
-type serialized_proof = bytes
+type serialized_proof = string
 
-let serialized_proof_encoding = Data_encoding.bytes
+let serialized_proof_encoding = Data_encoding.(string' Hex)
 
 module type Merkelized_operations = sig
   type inbox_context
@@ -758,9 +758,9 @@ struct
             Next_level {lower_message_proof; inc});
       ]
 
-  let of_serialized_proof = Data_encoding.Binary.of_bytes_opt proof_encoding
+  let of_serialized_proof = Data_encoding.Binary.of_string_opt proof_encoding
 
-  let to_serialized_proof = Data_encoding.Binary.to_bytes_exn proof_encoding
+  let to_serialized_proof = Data_encoding.Binary.to_string_exn proof_encoding
 
   let proof_error reason =
     let open Lwt_result_syntax in
@@ -935,7 +935,7 @@ struct
       |> Option.map (lift_ptr_path deref)
       |> Option.join |> return
 
-    let serialized_proof_of_string x = Bytes.of_string x
+    let serialized_proof_of_string x = x
 
     let inbox_message_counter = inbox_message_counter
   end
diff --git a/src/proto_alpha/lib_protocol/sc_rollup_proof_repr.ml b/src/proto_alpha/lib_protocol/sc_rollup_proof_repr.ml
index bdcf201620c5aa03d5c4ad500b50f79b3142dad3..2dfe7e8c3cb6e211c77fc2e82ea7a8af8c0ba996 100644
--- a/src/proto_alpha/lib_protocol/sc_rollup_proof_repr.ml
+++ b/src/proto_alpha/lib_protocol/sc_rollup_proof_repr.ml
@@ -67,9 +67,11 @@ let reveal_proof_encoding =
          (req "reveal_proof_kind" (constant "raw_data_proof"))
          (req
             "raw_data"
-            (check_size Constants_repr.sc_rollup_message_size_limit bytes)))
-      (function Raw_data_proof s -> Some ((), Bytes.of_string s) | _ -> None)
-      (fun ((), s) -> Raw_data_proof (Bytes.to_string s))
+            (check_size
+               Constants_repr.sc_rollup_message_size_limit
+               (string' Hex))))
+      (function Raw_data_proof s -> Some ((), s) | _ -> None)
+      (fun ((), s) -> Raw_data_proof s)
   and case_metadata_proof =
     case
       ~title:"metadata proof"
diff --git a/src/proto_alpha/lib_protocol/tx_rollup_l2_proof.ml b/src/proto_alpha/lib_protocol/tx_rollup_l2_proof.ml
index 2b0010fe85305131c81fb27e9cef2ead635f5bb9..806ab8da15eecf2cb5fc7e236ec2733e834248ac 100644
--- a/src/proto_alpha/lib_protocol/tx_rollup_l2_proof.ml
+++ b/src/proto_alpha/lib_protocol/tx_rollup_l2_proof.ml
@@ -32,11 +32,7 @@ type serialized = string
 
 let length = String.length
 
-let serialized_encoding =
-  let open Data_encoding in
-  (* Deal with unprintable string *)
-  let json = conv Bytes.of_string Bytes.to_string bytes in
-  splitted ~json ~binary:string
+let serialized_encoding = Data_encoding.(string' Hex)
 
 let proof_of_serialized_opt = Data_encoding.Binary.of_string_opt encoding
 
diff --git a/src/proto_alpha/lib_protocol/tx_rollup_message_repr.ml b/src/proto_alpha/lib_protocol/tx_rollup_message_repr.ml
index a393b22676f9bfafe2e2dd6b31624b1646410891..c86c770d68dcb9d52ab7b3956d347390b86e1a68 100644
--- a/src/proto_alpha/lib_protocol/tx_rollup_message_repr.ml
+++ b/src/proto_alpha/lib_protocol/tx_rollup_message_repr.ml
@@ -45,11 +45,6 @@ let deposit_encoding =
        (req "ticket_hash" Ticket_hash_repr.encoding)
        (req "amount" Tx_rollup_l2_qty.encoding)
 
-let batch_encoding =
-  let open Data_encoding in
-  let json = conv Bytes.of_string Bytes.to_string bytes in
-  splitted ~json ~binary:string
-
 type t = Batch of string | Deposit of deposit
 
 let encoding =
@@ -60,7 +55,7 @@ let encoding =
       case
         (Tag 0)
         ~title:"Batch"
-        (obj1 (req "batch" batch_encoding))
+        (obj1 (req "batch" (string' Hex)))
         (function Batch batch -> Some batch | _ -> None)
         (fun batch -> Batch batch);
       case
diff --git a/src/proto_alpha/lib_sc_rollup/l2_message.ml b/src/proto_alpha/lib_sc_rollup/l2_message.ml
index 9b669451cd09897575770c2a312b2deaba20f202..71987dc50b8dfbfd6dc0a91d3c7916e3fedac529 100644
--- a/src/proto_alpha/lib_sc_rollup/l2_message.ml
+++ b/src/proto_alpha/lib_sc_rollup/l2_message.ml
@@ -33,7 +33,7 @@ type t = {
 let content_encoding =
   let open Data_encoding in
   def "sc_l2_message" ~description:"A hex encoded smart contact rollup message"
-  @@ conv String.to_bytes String.of_bytes bytes
+  @@ string' Hex
 
 let encoding =
   let open Data_encoding in
diff --git a/tests_python/tests_alpha/test_basic.py b/tests_python/tests_alpha/test_basic.py
index 3a0a97f40cb56bd437eaccf2fdccd63fc0d7badf..b5702c307f92aa5025bc79810f81aec8ef44d9b8 100644
--- a/tests_python/tests_alpha/test_basic.py
+++ b/tests_python/tests_alpha/test_basic.py
@@ -143,7 +143,12 @@ class TestRawContext:
         run_json = {
             'operation': {
                 "branch": head_hash,
-                "contents": [{"kind": "failing_noop", "arbitrary": message}],
+                "contents": [
+                    {
+                        "kind": "failing_noop",
+                        "arbitrary": bytes(message, 'utf-8').hex(),
+                    }
+                ],
                 'signature': signature,
             },
             'chain_id': chain_id,