diff --git a/Tests/test_file_jpeg2k.py b/Tests/test_file_jpeg2k.py
index 2ef262e3e7f..ca410162a1c 100644
--- a/Tests/test_file_jpeg2k.py
+++ b/Tests/test_file_jpeg2k.py
@@ -10,7 +10,6 @@
     assert_image_equal,
     assert_image_similar,
     assert_image_similar_tofile,
-    is_big_endian,
     skip_unless_feature,
 )
 
@@ -234,13 +233,11 @@ def test_16bit_monochrome_has_correct_mode():
         assert jp2.mode == "I;16"
 
 
-@pytest.mark.xfail(is_big_endian(), reason="Fails on big-endian")
 def test_16bit_monochrome_jp2_like_tiff():
     with Image.open("Tests/images/16bit.cropped.tif") as tiff_16bit:
         assert_image_similar_tofile(tiff_16bit, "Tests/images/16bit.cropped.jp2", 1e-3)
 
 
-@pytest.mark.xfail(is_big_endian(), reason="Fails on big-endian")
 def test_16bit_monochrome_j2k_like_tiff():
     with Image.open("Tests/images/16bit.cropped.tif") as tiff_16bit:
         assert_image_similar_tofile(tiff_16bit, "Tests/images/16bit.cropped.j2k", 1e-3)
diff --git a/Tests/test_file_png.py b/Tests/test_file_png.py
index 9a5577bbac0..0869cc58bc5 100644
--- a/Tests/test_file_png.py
+++ b/Tests/test_file_png.py
@@ -13,7 +13,6 @@
     assert_image_equal,
     assert_image_equal_tofile,
     hopper,
-    is_big_endian,
     is_win32,
     mark_if_feature_version,
     skip_unless_feature,
@@ -77,7 +76,6 @@ def get_chunks(self, filename):
                     png.crc(cid, s)
         return chunks
 
-    @pytest.mark.xfail(is_big_endian(), reason="Fails on big-endian")
     def test_sanity(self, tmp_path):
 
         # internal version number
diff --git a/src/libImaging/Jpeg2KDecode.c b/src/libImaging/Jpeg2KDecode.c
index 601bd4b6206..8f27d87d88c 100644
--- a/src/libImaging/Jpeg2KDecode.c
+++ b/src/libImaging/Jpeg2KDecode.c
@@ -180,9 +180,11 @@ j2ku_gray_i(
         case 2:
             for (y = 0; y < h; ++y) {
                 const UINT16 *data = (const UINT16 *)&tiledata[2 * y * w];
-                UINT16 *row = (UINT16 *)im->image[y0 + y] + x0;
+                UINT8 *row = (UINT8 *)im->image[y0 + y] + x0;
                 for (x = 0; x < w; ++x) {
-                    *row++ = j2ku_shift(offset + *data++, shift);
+                    UINT16 pixel = j2ku_shift(offset + *data++, shift);
+                    *row++ = pixel;
+                    *row++ = pixel >> 8;
                 }
             }
             break;
diff --git a/src/libImaging/Jpeg2KEncode.c b/src/libImaging/Jpeg2KEncode.c
index 70185315999..86cd7d5af26 100644
--- a/src/libImaging/Jpeg2KEncode.c
+++ b/src/libImaging/Jpeg2KEncode.c
@@ -110,8 +110,15 @@ j2k_pack_i16(Imaging im, UINT8 *buf, unsigned x0, unsigned y0, unsigned w, unsig
     for (y = 0; y < h; ++y) {
         UINT8 *data = (UINT8 *)(im->image[y + y0] + x0);
         for (x = 0; x < w; ++x) {
-            *ptr++ = *data++;
-            *ptr++ = *data++;
+#ifdef WORDS_BIGENDIAN
+            ptr[0] = data[1];
+            ptr[1] = data[0];
+#else
+            ptr[0] = data[0];
+            ptr[1] = data[1];
+#endif
+            ptr += 2;
+            data += 2;
         }
     }
 }
@@ -301,13 +308,7 @@ j2k_encode_entry(Imaging im, ImagingCodecState state) {
         components = 1;
         color_space = OPJ_CLRSPC_GRAY;
         pack = j2k_pack_l;
-    } else if (strcmp(im->mode, "I;16") == 0) {
-        components = 1;
-        color_space = OPJ_CLRSPC_GRAY;
-        pack = j2k_pack_i16;
-        prec = 16;
-        bpp = 12;
-    } else if (strcmp(im->mode, "I;16B") == 0) {
+    } else if (strcmp(im->mode, "I;16") == 0 || strcmp(im->mode, "I;16B") == 0) {
         components = 1;
         color_space = OPJ_CLRSPC_GRAY;
         pack = j2k_pack_i16;
diff --git a/src/libImaging/Pack.c b/src/libImaging/Pack.c
index 2fdee919f6c..0c7c0497efe 100644
--- a/src/libImaging/Pack.c
+++ b/src/libImaging/Pack.c
@@ -656,7 +656,11 @@ static struct {
 
     /* storage modes */
     {"I;16", "I;16", 16, copy2},
+#ifdef WORDS_BIGENDIAN
+    {"I;16", "I;16B", 16, packI16N_I16},
+#else
     {"I;16", "I;16B", 16, packI16N_I16B},
+#endif
     {"I;16B", "I;16B", 16, copy2},
     {"I;16L", "I;16L", 16, copy2},
     {"I;16", "I;16N", 16, packI16N_I16},  // LibTiff native->image endian.