diff -r 79d4c281a63b -r 9170a7dff573 tests/test_list.c
--- a/tests/test_list.c	Fri Nov 07 18:42:06 2025 +0100
+++ b/tests/test_list.c	Fri Nov 07 19:13:28 2025 +0100
@@ -2872,6 +2872,110 @@
     }
 }
 
+static CX_TEST_SUBROUTINE(verify_union, bool sorted, bool alloc_fail) {
+    CxTestingAllocator talloc;
+    cx_testing_allocator_init(&talloc);
+
+    CxList *dst = cxLinkedListCreate(cxDefaultAllocator, cx_cmp_int, CX_STORE_POINTERS);
+    cxDefineAdvancedDestructor(dst, cxFree, &talloc);
+    CxList *src = cxLinkedListCreate(cxDefaultAllocator, cx_cmp_int, sizeof(int));
+    CxList *other = cxLinkedListCreate(cxDefaultAllocator, cx_cmp_int, sizeof(int));
+
+    int dst_data[] = {47, 178, 176, 83};
+    int src_data[] = {
+        153, 106, 171, 130, 74, 173, 150, 94, 27, 92, 70, 175, 200, 20, 29, 161, 88, 116, 71, 53, 199, 124, 32, 9, 76,
+        151, 33, 51, 37, 65, 176, 49, 12, 162, 28, 85, 4, 177, 198, 54, 109, 188, 44, 77, 194, 63, 41, 129, 97, 83
+    };
+    int other_data[] = {
+        75, 137, 176, 111, 85, 27, 197, 141, 46, 103, 69, 146, 49, 79, 63, 130, 154, 45, 38, 139, 193, 90, 64, 142, 115,
+        120, 78, 100, 101, 42, 21, 1, 161, 10, 114, 198, 181, 178, 136, 188, 59, 41, 73, 99, 151, 144, 118, 53, 199, 71
+    };
+
+    for (unsigned i = 0 ; i < cx_nmemb(dst_data) ; i++) {
+        int *x = cxMalloc(&talloc.base, sizeof(int));
+        *x = dst_data[i];
+        cxListAdd(dst, x);
+    }
+    cxListAddArray(src, src_data, 50);
+    cxListAddArray(other, other_data, 50);
+    if (sorted) {
+        cxListSort(dst);
+        cxListSort(src);
+        cxListSort(other);
+    }
+
+    // the 14 elements from the intersection should not appear twice in the destination:
+    // 27, 41, 49, 53, 63, 71, 85, 130, 151, 161, 176, 188, 198, 199
+    // however, the elements that are already in dst may appear twice
+    size_t expected_len = 90;
+    int expected_unsorted[] = {
+        47, 178, 176, 83,
+        153, 106, 171, 130, 74, 173, 150, 94, 27, 92, 70, 175, 200, 20, 29, 161,
+        88, 116, 71, 53, 199, 124, 32, 9, 76, 151, 33, 51, 37, 65, 176, 49, 12,
+        162, 28, 85, 4, 177, 198, 54, 109, 188, 44, 77, 194, 63, 41, 129, 97,
+        83, 75, 137, 111, 197, 141, 46, 103, 69, 146, 79, 154, 45, 38, 139, 193,
+        90, 64, 142, 115, 120, 78, 100, 101, 42, 21, 1, 10, 114, 181, 178, 136,
+        59, 73, 99, 144, 118
+    };
+    int expected_sorted[] = {
+        47, 83, 176, 178,
+        1, 4, 9, 10, 12, 20, 21, 27, 28, 29, 32, 33, 37, 38, 41, 42, 44, 45,
+        46, 49, 51, 53, 54, 59, 63, 64, 65, 69, 70, 71, 73, 74, 75, 76, 77, 78,
+        79, 83, 85, 88, 90, 92, 94, 97, 99, 100, 101, 103, 106, 109, 111, 114,
+        115, 116, 118, 120, 124, 129, 130, 136, 137, 139, 141, 142, 144, 146,
+        150, 151, 153, 154, 161, 162, 171, 173, 175, 176, 177, 178, 181, 188,
+        193, 194, 197, 198, 199, 200
+    };
+
+    if (alloc_fail) {
+        test_clone_func_max_enabled = true;
+        test_clone_func_max_clones = 66;
+        expected_len = 70;
+    }
+    CxList *expected = cxArrayListCreate(cxDefaultAllocator, cx_cmp_int, sizeof(int), expected_len);
+    cxListAddArray(expected, sorted ? expected_sorted : expected_unsorted, expected_len);
+
+    int result = cxListUnion(dst, src, other, test_clone_func, &talloc.base, NULL);
+    if (alloc_fail) {
+        CX_TEST_ASSERT(result != 0);
+    } else {
+        CX_TEST_ASSERT(result == 0);
+    }
+    test_clone_func_max_enabled = false;
+    CX_TEST_ASSERT(expected_len == cxListSize(dst));
+    CX_TEST_ASSERT(0 == cxListCompare(dst, expected));
+
+    cxListFree(dst);
+    cxListFree(src);
+    cxListFree(other);
+    cxListFree(expected);
+    CX_TEST_ASSERT(cx_testing_allocator_verify(&talloc));
+}
+
+CX_TEST(test_list_union_unsorted) {
+    CX_TEST_DO {
+        CX_TEST_CALL_SUBROUTINE(verify_union, false, false);
+    }
+}
+
+CX_TEST(test_list_union_sorted) {
+    CX_TEST_DO {
+        CX_TEST_CALL_SUBROUTINE(verify_union, true, false);
+    }
+}
+
+CX_TEST(test_list_union_unsorted_alloc_fail) {
+    CX_TEST_DO {
+        CX_TEST_CALL_SUBROUTINE(verify_union, false, true);
+    }
+}
+
+CX_TEST(test_list_union_sorted_alloc_fail) {
+    CX_TEST_DO {
+        CX_TEST_CALL_SUBROUTINE(verify_union, true, true);
+    }
+}
+
 CX_TEST(test_list_pointer_list_supports_null) {
     CxList *list = cxLinkedListCreate(cxDefaultAllocator, cx_cmp_int, CX_STORE_POINTERS);
     int x = 47;
@@ -3327,6 +3431,10 @@
     CxTestSuite *suite = cx_test_suite_new("list collection operations");
 
     // we do not perform the following tests with every combination of list types
+    cx_test_register(suite, test_list_union_unsorted);
+    cx_test_register(suite, test_list_union_sorted);
+    cx_test_register(suite, test_list_union_unsorted_alloc_fail);
+    cx_test_register(suite, test_list_union_sorted_alloc_fail);
     cx_test_register(suite, test_list_difference_unsorted);
     cx_test_register(suite, test_list_difference_sorted);
     cx_test_register(suite, test_list_difference_unsorted_alloc_fail);