diff --git a/packages/adapter/src/io-ts.ts b/packages/adapter/src/io-ts.ts
index f685e202..8084f3da 100644
--- a/packages/adapter/src/io-ts.ts
+++ b/packages/adapter/src/io-ts.ts
@@ -150,21 +150,25 @@ export interface BrandP<A, B extends A, I>
 // This helper is a workaround to that issue, making sure that if the original codec
 // has the same Output than the Actual type, the new codec outputs the branded type
 // So a PositiveInt codec will have the type t.Type<PositiveInt, PositiveInt, unknown>
-export function preservedBrand<A, I, B extends A>(
-  codec: t.Type<A, A, I>,
-  predicate: Refinement<A, B>,
-  name: string
-): BrandP<A, B, I> {
+export function preservedBrand<
+  C extends t.Any,
+  N extends string,
+  B extends { readonly [K in N]: symbol },
+>(
+  codec: C,
+  predicate: Refinement<t.TypeOf<C>, t.Branded<t.TypeOf<C>, B>>,
+  name: N
+): BrandP<t.TypeOf<C>, B, t.InputOf<C>> {
   return new t.Type(
     name,
-    (u): u is t.Branded<A, B> => codec.is(u) && predicate(u),
+    (u): u is t.Branded<t.TypeOf<C>, B> => codec.is(u) && predicate(u),
     (u, c) =>
       pipe(
         codec.validate(u, c),
         Either.chain((a) =>
           predicate(a)
-            ? t.success(a as t.Branded<A, B>)
-            : t.failure<t.Branded<A, B>>(
+            ? t.success(a as t.Branded<t.TypeOf<C>, B>)
+            : t.failure<t.Branded<t.TypeOf<C>, B>>(
                 a,
                 c,
                 `Value does not satisfy the ${name} constraint`
diff --git a/packages/runtime/client/rest/src/contract/transaction/details.ts b/packages/runtime/client/rest/src/contract/transaction/details.ts
index 98ec20ea..540c3fdf 100644
--- a/packages/runtime/client/rest/src/contract/transaction/details.ts
+++ b/packages/runtime/client/rest/src/contract/transaction/details.ts
@@ -23,6 +23,7 @@ import {
   BlockHeader,
   ContractIdGuard,
   TextEnvelope,
+  TxIdGuard,
 } from "@marlowe.io/runtime-core";
 import { TxStatus } from "./status.js";
 import { assertGuardEqual, proxy } from "@marlowe.io/adapter/io-ts";
@@ -97,7 +98,7 @@ export const TransactionDetailsGuard = assertGuardEqual(
   proxy<TransactionDetails>(),
   t.type({
     contractId: ContractIdGuard,
-    transactionId: TxId,
+    transactionId: TxIdGuard,
     continuations: optionFromNullable(G.BuiltinByteString),
     tags: TagsGuard,
     metadata: MetadataGuard,
@@ -108,7 +109,7 @@ export const TransactionDetailsGuard = assertGuardEqual(
     outputUtxo: optionFromNullable(TxOutRef),
     outputContract: optionFromNullable(G.Contract),
     outputState: optionFromNullable(G.MarloweState),
-    consumingTx: optionFromNullable(TxId),
+    consumingTx: optionFromNullable(TxIdGuard),
     invalidBefore: ISO8601,
     invalidHereafter: ISO8601,
     txBody: optionFromNullable(TextEnvelopeGuard),
diff --git a/packages/runtime/client/rest/src/contract/transaction/endpoints/collection.ts b/packages/runtime/client/rest/src/contract/transaction/endpoints/collection.ts
index 8536b6d2..da74487b 100644
--- a/packages/runtime/client/rest/src/contract/transaction/endpoints/collection.ts
+++ b/packages/runtime/client/rest/src/contract/transaction/endpoints/collection.ts
@@ -27,6 +27,7 @@ import {
   unTxOutRef,
   ContractId,
   ContractIdGuard,
+  TxIdGuard,
 } from "@marlowe.io/runtime-core";
 import { TxHeader, TxHeaderGuard } from "../header.js";
 import { assertGuardEqual, proxy } from "@marlowe.io/adapter/io-ts";
@@ -125,7 +126,7 @@ export const GetTransactionsForContractResponseGuard = assertGuardEqual(
 export type TransactionTextEnvelope = t.TypeOf<typeof TransactionTextEnvelope>;
 export const TransactionTextEnvelope = t.type({
   contractId: ContractIdGuard,
-  transactionId: TxId,
+  transactionId: TxIdGuard,
   tx: TextEnvelopeGuard,
 });
 
diff --git a/packages/runtime/client/rest/src/contract/transaction/endpoints/singleton.ts b/packages/runtime/client/rest/src/contract/transaction/endpoints/singleton.ts
index 7f3e6b45..aa0fcaa2 100644
--- a/packages/runtime/client/rest/src/contract/transaction/endpoints/singleton.ts
+++ b/packages/runtime/client/rest/src/contract/transaction/endpoints/singleton.ts
@@ -14,6 +14,7 @@ import {
   HexTransactionWitnessSet,
   HexTransactionWitnessSetGuard,
   TxId,
+  TxIdGuard,
   transactionWitnessSetTextEnvelope,
 } from "@marlowe.io/runtime-core";
 
@@ -45,7 +46,7 @@ export const GetContractTransactionByIdRequestGuard = assertGuardEqual(
   proxy<GetContractTransactionByIdRequest>(),
   t.type({
     contractId: ContractIdGuard,
-    txId: TxId,
+    txId: TxIdGuard,
   })
 );
 
@@ -80,7 +81,7 @@ export const SubmitContractTransactionRequestGuard = assertGuardEqual(
   proxy<SubmitContractTransactionRequest>(),
   t.type({
     contractId: ContractIdGuard,
-    transactionId: TxId,
+    transactionId: TxIdGuard,
     hexTransactionWitnessSet: HexTransactionWitnessSetGuard,
   })
 );
diff --git a/packages/runtime/client/rest/src/contract/transaction/header.ts b/packages/runtime/client/rest/src/contract/transaction/header.ts
index b25c8f52..1cd02863 100644
--- a/packages/runtime/client/rest/src/contract/transaction/header.ts
+++ b/packages/runtime/client/rest/src/contract/transaction/header.ts
@@ -14,6 +14,7 @@ import {
   TxOutRef,
   TxId,
   ContractId,
+  TxIdGuard,
 } from "@marlowe.io/runtime-core";
 import { TxStatus } from "./status.js";
 import { BuiltinByteString } from "@marlowe.io/language-core-v1";
@@ -46,7 +47,7 @@ export interface TxHeader {
  */
 export const TxHeaderGuard = t.type({
   contractId: ContractIdGuard,
-  transactionId: TxId,
+  transactionId: TxIdGuard,
   continuations: optionFromNullable(G.BuiltinByteString),
   tags: TagsGuard,
   metadata: MetadataGuard,
diff --git a/packages/runtime/core/src/contract/id.ts b/packages/runtime/core/src/contract/id.ts
index d8d2f8d9..15b9f8e0 100644
--- a/packages/runtime/core/src/contract/id.ts
+++ b/packages/runtime/core/src/contract/id.ts
@@ -2,7 +2,7 @@ import * as t from "io-ts/lib/index.js";
 import { split } from "fp-ts/lib/string.js";
 import { pipe } from "fp-ts/lib/function.js";
 import { head } from "fp-ts/lib/ReadonlyNonEmptyArray.js";
-import { TxId } from "../tx/id.js";
+import { TxId, txId } from "../tx/id.js";
 import { unsafeEither } from "@marlowe.io/adapter/fp-ts";
 import { preservedBrand } from "@marlowe.io/adapter/io-ts";
 
@@ -16,11 +16,17 @@ export const ContractIdGuard = preservedBrand(
   "ContractId"
 );
 
-export type ContractId = t.TypeOf<typeof ContractIdGuard>;
+/**
+ * Marlowe contract identifier.
+ *
+ * @remarks The underlying data structure is a normal string, but in the type
+ * level it is **Branded** with a unique symbol so that it is not confused with other strings
+ */
+export type ContractId = t.Branded<string, ContractIdBrand>;
 
 export const contractId = (s: string) =>
   unsafeEither(ContractIdGuard.decode(s));
 
 export const contractIdToTxId: (contractId: ContractId) => TxId = (
   contractId
-) => pipe(contractId, split("#"), head);
+) => pipe(contractId, split("#"), head, txId);
diff --git a/packages/runtime/core/src/payout/index.ts b/packages/runtime/core/src/payout/index.ts
index 7ef009d1..63fe2c9c 100644
--- a/packages/runtime/core/src/payout/index.ts
+++ b/packages/runtime/core/src/payout/index.ts
@@ -4,7 +4,7 @@ import { fromNewtype } from "io-ts-types";
 import { split } from "fp-ts/lib/string.js";
 import { pipe } from "fp-ts/lib/function.js";
 import { head } from "fp-ts/lib/ReadonlyNonEmptyArray.js";
-import { TxId } from "../tx/id.js";
+import { txId, TxId } from "../tx/id.js";
 import { ContractIdGuard } from "../contract/id.js";
 import { AssetId, Assets } from "../asset/index.js";
 
@@ -15,7 +15,7 @@ export const unPayoutId = iso<PayoutId>().unwrap;
 export const payoutId = iso<PayoutId>().wrap;
 
 export const payoutIdToTxId: (payoutId: PayoutId) => TxId = (payoutId) =>
-  pipe(payoutId, unPayoutId, split("#"), head);
+  pipe(payoutId, unPayoutId, split("#"), head, txId);
 
 export type WithdrawalId = Newtype<
   { readonly WithdrawalId: unique symbol },
@@ -27,7 +27,7 @@ export const withdrawalId = iso<WithdrawalId>().wrap;
 
 export const withdrawalIdToTxId: (withdrawalId: WithdrawalId) => TxId = (
   withdrawalId
-) => pipe(withdrawalId, unWithdrawalId);
+) => pipe(withdrawalId, unWithdrawalId, txId);
 
 // DISCUSSION: PayoutAvailable or AvailablePayout?
 export type PayoutAvailable = t.TypeOf<typeof PayoutAvailable>;
diff --git a/packages/runtime/core/src/tx/id.ts b/packages/runtime/core/src/tx/id.ts
index 2dcea5bc..63399c90 100644
--- a/packages/runtime/core/src/tx/id.ts
+++ b/packages/runtime/core/src/tx/id.ts
@@ -1,6 +1,22 @@
+import { unsafeEither } from "@marlowe.io/adapter/fp-ts";
 import * as t from "io-ts/lib/index.js";
 
-// TODO: Try to make newtype as this gets replaced to string
-//       in the docs.
-export type TxId = t.TypeOf<typeof TxId>;
-export const TxId = t.string; // to refine
+export interface TxIdBrand {
+  readonly TxId: unique symbol;
+}
+
+/**
+ * Cardano transaction identifier.
+ *
+ * @remarks The underlying data structure is a normal string, but in the type
+ * level it is **Branded** with a unique symbol so that it is not confused with other strings
+ */
+export type TxId = t.Branded<string, TxIdBrand>;
+
+export const TxIdGuard = t.brand(
+  t.string,
+  (s): s is t.Branded<string, TxIdBrand> => true,
+  "TxId"
+);
+
+export const txId = (s: string) => unsafeEither(TxIdGuard.decode(s));
diff --git a/typedoc.json b/typedoc.json
index c2116154..86e8a00c 100644
--- a/typedoc.json
+++ b/typedoc.json
@@ -24,6 +24,9 @@
     "fp-ts": {
       "Option": "https://gcanti.github.io/fp-ts/modules/Option.ts.html"
     },
+    "io-ts": {
+      "Branded": "https://github.com/gcanti/io-ts/blob/master/index.md#branded-types--refinements"
+    },
     "lucid-cardano": {
       "Lucid": "https://deno.land/x/lucid@0.10.7/mod.ts?s=Lucid"
     },