diff --git a/gobject/gatomicarray.c b/gobject/gatomicarray.c
index 506c666c2..57807f1d7 100644
--- a/gobject/gatomicarray.c
+++ b/gobject/gatomicarray.c
@@ -76,13 +76,14 @@ freelist_alloc (gsize size, gboolean reuse)
 	}
     }
 
-  real_size = sizeof (gsize) + MAX (size, sizeof (FreeListNode));
+  real_size = sizeof (GAtomicArrayMetadata) + MAX (size, sizeof (FreeListNode));
   mem = g_slice_alloc (real_size);
-  mem = ((char *) mem) + sizeof (gsize);
+  mem = ((char *) mem) + sizeof (GAtomicArrayMetadata);
   G_ATOMIC_ARRAY_DATA_SIZE (mem) = size;
 
 #if ENABLE_VALGRIND
-  VALGRIND_MALLOCLIKE_BLOCK (mem, real_size - sizeof (gsize), FALSE, FALSE);
+  VALGRIND_MALLOCLIKE_BLOCK (mem, real_size - sizeof (GAtomicArrayMetadata),
+                             FALSE, FALSE);
 #endif
 
   return mem;
diff --git a/gobject/gatomicarray.h b/gobject/gatomicarray.h
index f7761cc65..a4fdc0103 100644
--- a/gobject/gatomicarray.h
+++ b/gobject/gatomicarray.h
@@ -27,7 +27,16 @@
 
 G_BEGIN_DECLS
 
-#define G_ATOMIC_ARRAY_DATA_SIZE(mem) (*((gsize *) (mem) - 1))
+typedef union _GAtomicArrayMetadata
+{
+  gsize size;
+  /* We have to ensure that the memory location is sufficiently aligned to
+   * store any object. With C11 this would be max_align_t, but in practise
+   * gpointer is sufficient for all known architectures. We could change
+   * this to `_Alignas(max_align_t) char pad` once we depend on C11. */
+  gpointer _alignment_padding;
+} GAtomicArrayMetadata;
+#define G_ATOMIC_ARRAY_DATA_SIZE(mem) (((GAtomicArrayMetadata *) (mem) - 1)->size)
 
 typedef struct _GAtomicArray GAtomicArray;
 struct _GAtomicArray {