diff --git a/test/built-ins/Temporal/Calendar/prototype/mergeFields/basic.js b/test/built-ins/Temporal/Calendar/prototype/mergeFields/basic.js
new file mode 100644
index 00000000000..72483362e8b
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/mergeFields/basic.js
@@ -0,0 +1,33 @@
+// Copyright (C) 2021 the V8 project authors. All rights reserved.
+// Copyright (C) 2022 Igalia S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.mergefields
+description: >
+  Temporal.Calendar.prototype.mergeFields will merge own data properties on its
+  arguments
+info: |
+  1. Let calendar be the this value.
+  2. Perform ? RequireInternalSlot(calendar, [[InitializedTemporalCalendar]]).
+  3. Assert: calendar.[[Identifier]] is "iso8601".
+  4. Set fields to ? ToObject(fields).
+  5. Set additionalFields to ? ToObject(additionalFields).
+  6. Return ? DefaultMergeFields(fields, additionalFields).
+features: [Temporal]
+includes: [deepEqual.js]
+---*/
+
+const cal = new Temporal.Calendar("iso8601");
+
+assert.deepEqual(
+  cal.mergeFields({ a: 1, b: 2 }, { c: 3, d: 4 }),
+  { a: 1, b: 2, c: 3, d: 4 },
+  "properties are merged"
+);
+
+assert.deepEqual(
+  cal.mergeFields({ a: 1, b: 2 }, { b: 3, c: 4 }),
+  { a: 1, b: 3, c: 4 },
+  "property in additionalFields should overwrite one in fields"
+);
diff --git a/test/built-ins/Temporal/Calendar/prototype/mergeFields/iso8601-calendar-month-monthCode.js b/test/built-ins/Temporal/Calendar/prototype/mergeFields/iso8601-calendar-month-monthCode.js
new file mode 100644
index 00000000000..e322a791e13
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/mergeFields/iso8601-calendar-month-monthCode.js
@@ -0,0 +1,99 @@
+// Copyright (C) 2021 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.mergefields
+description: >
+  The default mergeFields algorithm from the ISO 8601 calendar should correctly
+  merge the month and monthCode properties
+info: |
+  1. Let calendar be the this value.
+  2. Perform ? RequireInternalSlot(calendar, [[InitializedTemporalCalendar]]).
+  3. Assert: calendar.[[Identifier]] is "iso8601".
+  4. Set fields to ? ToObject(fields).
+  5. Set additionalFields to ? ToObject(additionalFields).
+  6. Return ? DefaultMergeFields(fields, additionalFields).
+features: [Temporal]
+includes: [deepEqual.js]
+---*/
+
+const cal = new Temporal.Calendar("iso8601");
+
+assert.deepEqual(
+  cal.mergeFields({ a: 1, b: 2, month: 7 }, { b: 3, c: 4 }),
+  { a: 1, b: 3, c: 4, month: 7 },
+  "month is copied from fields"
+);
+assert.deepEqual(
+  cal.mergeFields({ a: 1, b: 2, monthCode: "M08" }, { b: 3, c: 4 }),
+  { a: 1, b: 3, c: 4, monthCode: "M08" },
+  "monthCode is copied from fields"
+);
+assert.deepEqual(
+  cal.mergeFields({ a: 1, b: 2, month: 7, monthCode: "M08" }, { b: 3, c: 4 }),
+  { a: 1, b: 3, c: 4, month: 7, monthCode: "M08" },
+  "both month and monthCode are copied from fields, no validation is performed"
+);
+
+assert.deepEqual(
+  cal.mergeFields({ a: 1, b: 2 }, { b: 3, c: 4, month: 5 }),
+  { a: 1, b: 3, c: 4, month: 5 },
+  "month is copied from additionalFields"
+);
+assert.deepEqual(
+  cal.mergeFields({ a: 1, b: 2 }, { b: 3, c: 4, monthCode: "M06" }),
+  { a: 1, b: 3, c: 4, monthCode: "M06" },
+  "monthCode is copied from additionalFields"
+);
+assert.deepEqual(
+  cal.mergeFields({ a: 1, b: 2 }, { b: 3, c: 4, month: 5, monthCode: "M06" }),
+  { a: 1, b: 3, c: 4, month: 5, monthCode: "M06" },
+  "both month and monthCode are copied from additionalFields, no validation is performed"
+);
+
+assert.deepEqual(
+  cal.mergeFields({ a: 1, b: 2, month: 7 }, { b: 3, c: 4, month: 5 }),
+  { a: 1, b: 3, c: 4, month: 5 },
+  "month from additionalFields overrides month from fields"
+);
+assert.deepEqual(
+  cal.mergeFields({ a: 1, b: 2, monthCode: "M07" }, { b: 3, c: 4, monthCode: "M05" }),
+  { a: 1, b: 3, c: 4, monthCode: "M05" },
+  "monthCode from additionalFields overrides monthCode from fields"
+);
+assert.deepEqual(
+  cal.mergeFields({ a: 1, b: 2, monthCode: "M07" }, { b: 3, c: 4, month: 6 }),
+  { a: 1, b: 3, c: 4, month: 6 },
+  "month's presence on additionalFields blocks monthCode from fields"
+);
+assert.deepEqual(
+  cal.mergeFields({ a: 1, b: 2, month: 7 }, { b: 3, c: 4, monthCode: "M06" }),
+  { a: 1, b: 3, c: 4, monthCode: "M06"},
+  "monthCode's presence on additionalFields blocks month from fields"
+);
+assert.deepEqual(
+  cal.mergeFields({ a: 1, b: 2, month: 7, monthCode: "M08" },{ b: 3, c: 4, month: 5 }),
+  { a: 1, b: 3, c: 4, month: 5 },
+  "month's presence on additionalFields blocks both month and monthCode from fields"
+);
+assert.deepEqual(
+  cal.mergeFields({ a: 1, b: 2, month: 7, monthCode: "M08" }, { b: 3, c: 4, monthCode: "M06" }),
+  { a: 1, b: 3, c: 4, monthCode: "M06" },
+  "monthCode's presence on additionalFields blocks both month and monthCode from fields"
+);
+
+assert.deepEqual(
+  cal.mergeFields({ a: 1, b: 2, month: 7 }, { b: 3, c: 4, month: 5, monthCode: "M06" }),
+  { a: 1, b: 3, c: 4, month: 5, monthCode: "M06" },
+  "both month and monthCode are copied from additionalFields even when fields has month"
+);
+assert.deepEqual(
+  cal.mergeFields({ a: 1, b: 2, monthCode: "M07" }, { b: 3, c: 4, month: 5, monthCode: "M06" }),
+  { a: 1, b: 3, c: 4, month: 5, monthCode: "M06" },
+  "both month and monthCode are copied from additionalFields even when fields has monthCode"
+);
+assert.deepEqual(
+  cal.mergeFields({ a: 1, b: 2, month: 7, monthCode: "M08" }, { b: 3, c: 4, month: 5, monthCode: "M06" }),
+  { a: 1, b: 3, c: 4, month: 5, monthCode: "M06" },
+  "both month and monthCode are copied from additionalFields even when fields has both month and monthCode"
+);
diff --git a/test/built-ins/Temporal/Calendar/prototype/mergeFields/non-string-properties.js b/test/built-ins/Temporal/Calendar/prototype/mergeFields/non-string-properties.js
new file mode 100644
index 00000000000..2908be90ba8
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/mergeFields/non-string-properties.js
@@ -0,0 +1,34 @@
+// Copyright (C) 2021 the V8 project authors. All rights reserved.
+// Copyright (C) 2022 Igalia S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.mergefields
+description: Only string keys from the arguments are merged
+info: |
+  1. Let calendar be the this value.
+  2. Perform ? RequireInternalSlot(calendar, [[InitializedTemporalCalendar]]).
+  3. Assert: calendar.[[Identifier]] is "iso8601".
+  4. Set fields to ? ToObject(fields).
+  5. Set additionalFields to ? ToObject(additionalFields).
+  6. Return ? DefaultMergeFields(fields, additionalFields).
+features: [Temporal]
+includes: [deepEqual.js]
+---*/
+
+const cal = new Temporal.Calendar("iso8601");
+
+assert.deepEqual(
+  cal.mergeFields({ 1: 2 }, { 3: 4 }),
+  { "1": 2, "3": 4 },
+  "number keys are actually string keys and are merged as such"
+);
+assert.deepEqual(
+  cal.mergeFields({ 1n: 2 }, { 2n: 4 }),
+  { "1": 2, "2": 4 },
+  "bigint keys are actually string keys and are merged as such"
+);
+
+const foo = Symbol("foo");
+const bar = Symbol("bar");
+assert.deepEqual(cal.mergeFields({ [foo]: 1 }, { [bar]: 2 }), {}, "symbol keys are not merged");