diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -1440,6 +1440,14 @@
 #  define _LIBCPP_GCC_DIAGNOSTIC_IGNORED(str)
 #endif
 
+#if defined(_AIX) && !defined(_LIBCPP_COMPILER_GCC)
+#define _LIBCPP_PACKED_BYTE_FOR_AIX _Pragma("pack(1)")
+#define _LIBCPP_PACKED_BYTE_FOR_AIX_END _Pragma("pack(pop)")
+#else
+#define _LIBCPP_PACKED_BYTE_FOR_AIX      /* empty */
+#define _LIBCPP_PACKED_BYTE_FOR_AIX_END  /* empty */
+#endif
+
 #endif // __cplusplus
 
 #endif // _LIBCPP_CONFIG
diff --git a/libcxx/include/__format/parser_std_format_spec.h b/libcxx/include/__format/parser_std_format_spec.h
--- a/libcxx/include/__format/parser_std_format_spec.h
+++ b/libcxx/include/__format/parser_std_format_spec.h
@@ -52,6 +52,7 @@
  * * The format-type filtering needs to be done post parsing in the parser
  *   derived from @ref __parser_std.
  */
+_LIBCPP_PACKED_BYTE_FOR_AIX
 class _LIBCPP_TYPE_VIS _Flags {
 public:
   enum class _LIBCPP_ENUM_VIS _Alignment : uint8_t {
@@ -109,6 +110,7 @@
 
   _Type __type{_Type::__default};
 };
+_LIBCPP_PACKED_BYTE_FOR_AIX_END
 
 namespace __detail {
 template <class _CharT>
diff --git a/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_bool.pass.cpp b/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_bool.pass.cpp
--- a/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_bool.pass.cpp
+++ b/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_bool.pass.cpp
@@ -8,7 +8,6 @@
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // UNSUPPORTED: libcpp-no-concepts
 // UNSUPPORTED: libcpp-has-no-incomplete-format
-// XFAIL: LIBCXX-AIX-FIXME
 
 // <format>
 
diff --git a/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_char.pass.cpp b/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_char.pass.cpp
--- a/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_char.pass.cpp
+++ b/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_char.pass.cpp
@@ -8,7 +8,6 @@
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // UNSUPPORTED: libcpp-no-concepts
 // UNSUPPORTED: libcpp-has-no-incomplete-format
-// XFAIL: LIBCXX-AIX-FIXME
 
 // <format>
 
diff --git a/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_integer.pass.cpp b/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_integer.pass.cpp
--- a/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_integer.pass.cpp
+++ b/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_integer.pass.cpp
@@ -8,7 +8,6 @@
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // UNSUPPORTED: libcpp-no-concepts
 // UNSUPPORTED: libcpp-has-no-incomplete-format
-// XFAIL: LIBCXX-AIX-FIXME
 
 // <format>
 
diff --git a/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_string.pass.cpp b/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_string.pass.cpp
--- a/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_string.pass.cpp
+++ b/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_string.pass.cpp
@@ -8,7 +8,6 @@
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // UNSUPPORTED: libcpp-no-concepts
 // UNSUPPORTED: libcpp-has-no-incomplete-format
-// XFAIL: LIBCXX-AIX-FIXME
 
 // <format>