diff --git a/source/set-optional.d.ts b/source/set-optional.d.ts
index 224004dcf..f2cfa9107 100644
--- a/source/set-optional.d.ts
+++ b/source/set-optional.d.ts
@@ -27,9 +27,11 @@ type SomeOptional = SetOptional<Foo, 'b' | 'c'>;
 @category Object
 */
 export type SetOptional<BaseType, Keys extends keyof BaseType> =
-	Simplify<
-	// Pick just the keys that are readonly from the base type.
-	Except<BaseType, Keys> &
-	// Pick the keys that should be mutable from the base type and make them mutable.
-	Partial<Pick<BaseType, Keys>>
-	>;
+	BaseType extends unknown // To distribute `BaseType` when it's a union type.
+		? Simplify<
+		// Pick just the keys that are readonly from the base type.
+		Except<BaseType, Keys> &
+		// Pick the keys that should be mutable from the base type and make them mutable.
+		Partial<Except<BaseType, Exclude<keyof BaseType, Keys>>>
+		>
+		: never;
diff --git a/source/set-readonly.d.ts b/source/set-readonly.d.ts
index f8f4aa90c..bb6b19084 100644
--- a/source/set-readonly.d.ts
+++ b/source/set-readonly.d.ts
@@ -33,6 +33,6 @@ export type SetReadonly<BaseType, Keys extends keyof BaseType> =
 	BaseType extends unknown
 		? Simplify<
 		Except<BaseType, Keys> &
-		Readonly<Pick<BaseType, Keys>>
+		Readonly<Except<BaseType, Exclude<keyof BaseType, Keys>>>
 		>
 		: never;
diff --git a/source/set-required.d.ts b/source/set-required.d.ts
index 735c227c4..d7b00f4bb 100644
--- a/source/set-required.d.ts
+++ b/source/set-required.d.ts
@@ -35,6 +35,6 @@ export type SetRequired<BaseType, Keys extends keyof BaseType> =
 		// Pick just the keys that are optional from the base type.
 		Except<BaseType, Keys> &
 		// Pick the keys that should be required from the base type and make them required.
-		Required<Pick<BaseType, Keys>>
+		Required<Except<BaseType, Exclude<keyof BaseType, Keys>>>
 		>
 		: never;
diff --git a/test-d/set-optional.ts b/test-d/set-optional.ts
index 649b4ec07..aa7f8d1ac 100644
--- a/test-d/set-optional.ts
+++ b/test-d/set-optional.ts
@@ -16,3 +16,19 @@ expectType<{a?: number; b?: string; c?: boolean}>(variation3);
 // Fail if type changes even if optional is right.
 declare const variation4: SetOptional<{a: number; b?: string; c: boolean}, 'b' | 'c'>;
 expectNotAssignable<{a: boolean; b?: string; c?: boolean}>(variation4);
+
+// Preserves readonly modifier.
+declare const variation5: SetOptional<{readonly a: number; readonly b?: string; c: boolean}, 'b' | 'c'>;
+expectType<{readonly a: number; readonly b?: string; c?: boolean}>(variation5);
+
+// Works with unions.
+declare const variation6: SetOptional<{readonly a: number; b: number; c: boolean} | {a: string; readonly b: string; d: boolean}, 'a' | 'b'>;
+expectType<{readonly a?: number; b?: number; c: boolean} | {a?: string; readonly b?: string; d: boolean}>(variation6);
+
+// Marks all keys as optional, if `Keys` is `any`.
+declare const variation7: SetOptional<{readonly a: number; b: string; c: boolean}, any>;
+expectType<{readonly a?: number; b?: string; c?: boolean}>(variation7);
+
+// Does nothing, if `Keys` is `never`.
+declare const variation8: SetOptional<{a?: number; readonly b?: string; readonly c: boolean}, never>;
+expectType<{a?: number; readonly b?: string; readonly c: boolean}>(variation8);
diff --git a/test-d/set-readonly.ts b/test-d/set-readonly.ts
index d3d7dcf01..0ba0b9884 100644
--- a/test-d/set-readonly.ts
+++ b/test-d/set-readonly.ts
@@ -16,3 +16,19 @@ expectType<{readonly a: number; readonly b?: string; readonly c: boolean}>(varia
 // Fail if type changes even if readonly is right.
 declare const variation4: SetReadonly<{a: number; readonly b: string; c: boolean}, 'b' | 'c'>;
 expectNotAssignable<{a: boolean; readonly b: string; readonly c: boolean}>(variation4);
+
+// Preserves optional modifier.
+declare const variation5: SetReadonly<{a?: number; readonly b?: string; c?: boolean}, 'b' | 'c'>;
+expectType<{a?: number; readonly b?: string; readonly c?: boolean}>(variation5);
+
+// Works with unions.
+declare const variation6: SetReadonly<{a?: number; b: number; c: boolean} | {a: string; b?: string; d: boolean}, 'a' | 'b'>;
+expectType<{readonly a?: number; readonly b: number; c: boolean} | {readonly a: string; readonly b?: string; d: boolean}>(variation6);
+
+// Marks all keys as readonly, if `Keys` is `any`.
+declare const variation7: SetReadonly<{a?: number; b: string; c: boolean}, any>;
+expectType<{readonly a?: number; readonly b: string; readonly c: boolean}>(variation7);
+
+// Does nothing, if `Keys` is `never`.
+declare const variation8: SetReadonly<{a: number; readonly b: string; readonly c: boolean}, never>;
+expectType<{a: number; readonly b: string; readonly c: boolean}>(variation8);
diff --git a/test-d/set-required.ts b/test-d/set-required.ts
index 4094e9cc9..6e51f0333 100644
--- a/test-d/set-required.ts
+++ b/test-d/set-required.ts
@@ -20,3 +20,19 @@ expectNotAssignable<{a?: boolean; b: string; c: boolean}>(variation4);
 // Update one required and one optional to required in a union.
 declare const variation5: SetRequired<{a?: '1'; b: string; c?: boolean} | {a?: '2'; b: string; c?: boolean}, 'a' | 'b'>;
 expectType<{a: '1'; b: string; c?: boolean} | {a: '2'; b: string; c?: boolean}>(variation5);
+
+// Preserves readonly modifier.
+declare const variation6: SetRequired<{readonly a?: number; readonly b: string; c?: boolean}, 'b' | 'c'>;
+expectType<{readonly a?: number; readonly b: string; c: boolean}>(variation6);
+
+// Works with unions.
+declare const variation7: SetRequired<{readonly a?: number; b?: number; c?: boolean} | {a?: string; readonly b?: string; d?: boolean}, 'a' | 'b'>;
+expectType<{readonly a: number; b: number; c?: boolean} | {a: string; readonly b: string; d?: boolean}>(variation7);
+
+// Marks all keys as required, if `Keys` is `any`.
+declare const variation8: SetRequired<{readonly a?: number; b?: string; c?: boolean}, any>;
+expectType<{readonly a: number; b: string; c: boolean}>(variation8);
+
+// Does nothing, if `Keys` is `never`.
+declare const variation9: SetRequired<{a?: number; readonly b?: string; readonly c: boolean}, never>;
+expectType<{a?: number; readonly b?: string; readonly c: boolean}>(variation9);