diff --git a/Tests/images/hopper.jxl b/Tests/images/hopper.jxl
deleted file mode 100644
index d89d3c267..000000000
Binary files a/Tests/images/hopper.jxl and /dev/null differ
diff --git a/Tests/images/hopper_jxl_bits.ppm b/Tests/images/hopper_jxl_bits.ppm
deleted file mode 100644
index 881aca333..000000000
Binary files a/Tests/images/hopper_jxl_bits.ppm and /dev/null differ
diff --git a/Tests/images/flower.jxl b/Tests/images/jxl/flower.jxl
similarity index 100%
rename from Tests/images/flower.jxl
rename to Tests/images/jxl/flower.jxl
diff --git a/Tests/images/flower2.jxl b/Tests/images/jxl/flower2.jxl
similarity index 100%
rename from Tests/images/flower2.jxl
rename to Tests/images/jxl/flower2.jxl
diff --git a/Tests/images/jxl/hopper.jxl b/Tests/images/jxl/hopper.jxl
new file mode 100644
index 000000000..3be85de64
Binary files /dev/null and b/Tests/images/jxl/hopper.jxl differ
diff --git a/Tests/images/jxl/hopper_bw_500.jxl b/Tests/images/jxl/hopper_bw_500.jxl
new file mode 100644
index 000000000..79ad8883f
Binary files /dev/null and b/Tests/images/jxl/hopper_bw_500.jxl differ
diff --git a/Tests/images/jxl/hopper_gray.jxl b/Tests/images/jxl/hopper_gray.jxl
new file mode 100644
index 000000000..2f50436da
Binary files /dev/null and b/Tests/images/jxl/hopper_gray.jxl differ
diff --git a/Tests/images/iss634.jxl b/Tests/images/jxl/iss634.jxl
similarity index 100%
rename from Tests/images/iss634.jxl
rename to Tests/images/jxl/iss634.jxl
diff --git a/Tests/images/transparent.jxl b/Tests/images/jxl/transparent.jxl
similarity index 100%
rename from Tests/images/transparent.jxl
rename to Tests/images/jxl/transparent.jxl
diff --git a/Tests/images/jxl/unknown_mode.jxl b/Tests/images/jxl/unknown_mode.jxl
new file mode 100644
index 000000000..8e8f8ede3
Binary files /dev/null and b/Tests/images/jxl/unknown_mode.jxl differ
diff --git a/Tests/test_file_jxl.py b/Tests/test_file_jxl.py
index faefed00c..86e718a05 100644
--- a/Tests/test_file_jxl.py
+++ b/Tests/test_file_jxl.py
@@ -1,10 +1,11 @@
 from __future__ import annotations
 
+import os
 import re
 
 import pytest
 
-from PIL import Image, JpegXlImagePlugin, features
+from PIL import Image, JpegXlImagePlugin, UnidentifiedImageError, features
 
 from .helper import assert_image_similar_tofile, skip_unless_feature
 
@@ -13,10 +14,6 @@ try:
 except ImportError:
     pass
 
-# cjxl v0.9.2 41b8cdab
-# hopper.jxl: cjxl hopper.png hopper.jxl -q 75 -e 8
-# 16_bit_binary.jxl: cjxl 16_bit_binary.pgm 16_bit_binary.jxl -q 100 -e 9
-
 
 class TestUnsupportedJpegXl:
     def test_unsupported(self, monkeypatch: pytest.MonkeyPatch) -> None:
@@ -24,7 +21,7 @@ class TestUnsupportedJpegXl:
 
         with pytest.raises(OSError):
             with pytest.warns(UserWarning, match="JXL support not installed"):
-                Image.open("Tests/images/hopper.jxl")
+                Image.open("Tests/images/jxl/hopper.jxl")
 
 
 @skip_unless_feature("jpegxl")
@@ -34,53 +31,34 @@ class TestFileJpegXl:
         assert version is not None
         assert re.search(r"\d+\.\d+\.\d+$", version)
 
-    def test_read_rgb(self) -> None:
-        """
-        Can we read an RGB mode JPEG XL file without error?
-        Does it have the bits we expect?
-        """
-
-        with Image.open("Tests/images/hopper.jxl") as im:
-            assert im.mode == "RGB"
-            assert im.size == (128, 128)
+    @pytest.mark.parametrize(
+        "mode, test_file",
+        (
+            ("1", "hopper_bw_500.png"),
+            ("L", "hopper_gray.jpg"),
+            ("I;16", "jxl/16bit_subcutaneous.cropped.png"),
+            ("RGB", "hopper.jpg"),
+            ("RGBA", "transparent.png"),
+        ),
+    )
+    def test_read(self, mode: str, test_file: str) -> None:
+        with Image.open(
+            "Tests/images/jxl/"
+            + os.path.splitext(os.path.basename(test_file))[0]
+            + ".jxl"
+        ) as im:
             assert im.format == "JPEG XL"
-            im.getdata()
+            assert im.mode == mode
 
-            # generated with:
-            # djxl hopper.jxl hopper_jxl_bits.ppm
-            assert_image_similar_tofile(im, "Tests/images/hopper_jxl_bits.ppm", 1)
+            assert_image_similar_tofile(im, "Tests/images/" + test_file, 1.9)
 
-    def test_read_rgba(self) -> None:
-        # Generated with `cjxl transparent.png transparent.jxl -q 100 -e 8`
-        with Image.open("Tests/images/transparent.jxl") as im:
-            assert im.mode == "RGBA"
-            assert im.size == (200, 150)
-            assert im.format == "JPEG XL"
-            im.getdata()
-
-            im.tobytes()
-
-            assert_image_similar_tofile(im, "Tests/images/transparent.png", 1)
-
-    def test_read_i16(self) -> None:
-        """
-        Can we read 16-bit Grayscale JPEG XL image?
-        """
-
-        with Image.open("Tests/images/jxl/16bit_subcutaneous.cropped.jxl") as im:
-            assert im.mode == "I;16"
-            assert im.size == (128, 64)
-            assert im.format == "JPEG XL"
-            im.getdata()
-
-            assert_image_similar_tofile(
-                im, "Tests/images/jxl/16bit_subcutaneous.cropped.png", 1
-            )
+    def test_unknown_mode(self) -> None:
+        with pytest.raises(UnidentifiedImageError):
+            Image.open("Tests/images/jxl/unknown_mode.jxl")
 
     def test_JpegXlDecode_with_invalid_args(self) -> None:
         """
         Calling decoder functions with no arguments should result in an error.
         """
-
         with pytest.raises(TypeError):
             _jpegxl.JpegXlDecoder()
diff --git a/Tests/test_file_jxl_animated.py b/Tests/test_file_jxl_animated.py
index ebb342d3e..f4283a71e 100644
--- a/Tests/test_file_jxl_animated.py
+++ b/Tests/test_file_jxl_animated.py
@@ -12,17 +12,17 @@ pytestmark = skip_unless_feature("jpegxl")
 def test_n_frames() -> None:
     """Ensure that jxl format sets n_frames and is_animated attributes correctly."""
 
-    with Image.open("Tests/images/hopper.jxl") as im:
+    with Image.open("Tests/images/jxl/hopper.jxl") as im:
         assert im.n_frames == 1
         assert not im.is_animated
 
-    with Image.open("Tests/images/iss634.jxl") as im:
+    with Image.open("Tests/images/jxl/iss634.jxl") as im:
         assert im.n_frames == 41
         assert im.is_animated
 
 
 def test_duration() -> None:
-    with Image.open("Tests/images/iss634.jxl") as im:
+    with Image.open("Tests/images/jxl/iss634.jxl") as im:
         assert im.info["duration"] == 70
         assert im.info["timestamp"] == 0
 
@@ -43,17 +43,17 @@ def test_seek() -> None:
             assert im1.is_animated
 
             # Traverse frames in reverse, checking timestamps and durations
-            total_dur = 0
+            total_duration = 0
             for frame in reversed(range(im1.n_frames)):
                 im1.seek(frame)
                 im2.seek(frame)
 
                 assert_image_equal(im1.convert("RGB"), im2.convert("RGB"))
 
-                total_dur += im1.info["duration"]
+                total_duration += im1.info["duration"]
                 assert im1.info["duration"] == im2.info["duration"]
                 assert im1.info["timestamp"] == im1.info["timestamp"]
-            assert total_dur == 8000
+            assert total_duration == 8000
 
             assert im1.tell() == 0
             assert im2.tell() == 0
@@ -65,7 +65,7 @@ def test_seek() -> None:
 
 
 def test_seek_errors() -> None:
-    with Image.open("Tests/images/iss634.jxl") as im:
+    with Image.open("Tests/images/jxl/iss634.jxl") as im:
         with pytest.raises(EOFError, match="attempt to seek outside sequence"):
             im.seek(-1)
 
diff --git a/Tests/test_file_jxl_metadata.py b/Tests/test_file_jxl_metadata.py
index c88483518..e9f25c3ee 100644
--- a/Tests/test_file_jxl_metadata.py
+++ b/Tests/test_file_jxl_metadata.py
@@ -27,7 +27,7 @@ except ImportError:
 
 
 def test_read_exif_metadata() -> None:
-    with Image.open("Tests/images/flower.jxl") as im:
+    with Image.open("Tests/images/jxl/flower.jxl") as im:
         assert im.format == "JPEG XL"
         exif_data = im.info["exif"]
 
@@ -44,7 +44,7 @@ def test_read_exif_metadata() -> None:
 
 
 def test_read_exif_metadata_without_prefix() -> None:
-    with Image.open("Tests/images/flower2.jxl") as im:
+    with Image.open("Tests/images/jxl/flower2.jxl") as im:
         # Assert prefix is not present
         assert im.info["exif"][:6] != b"Exif\x00\x00"
 
@@ -53,12 +53,12 @@ def test_read_exif_metadata_without_prefix() -> None:
 
 
 def test_read_icc_profile() -> None:
-    with Image.open("Tests/images/flower.jxl") as im:
+    with Image.open("Tests/images/jxl/flower.jxl") as im:
         assert "icc_profile" in im.info
 
 
 def test_getxmp() -> None:
-    with Image.open("Tests/images/flower.jxl") as im:
+    with Image.open("Tests/images/jxl/flower.jxl") as im:
         assert "xmp" not in im.info
         if ElementTree is None:
             with pytest.warns(
@@ -70,7 +70,7 @@ def test_getxmp() -> None:
             xmp = im.getxmp()
         assert xmp == {}
 
-    with Image.open("Tests/images/flower2.jxl") as im:
+    with Image.open("Tests/images/jxl/flower2.jxl") as im:
         if ElementTree is None:
             with pytest.warns(
                 UserWarning,
@@ -105,10 +105,10 @@ def test_4_byte_exif(monkeypatch: pytest.MonkeyPatch) -> None:
 
     monkeypatch.setattr(JpegXlImagePlugin, "_jpegxl", _mock_jpegxl)
 
-    with Image.open("Tests/images/hopper.jxl") as im:
+    with Image.open("Tests/images/jxl/hopper.jxl") as im:
         assert "exif" not in im.info
 
 
 def test_read_exif_metadata_empty() -> None:
-    with Image.open("Tests/images/hopper.jxl") as im:
+    with Image.open("Tests/images/jxl/hopper.jxl") as im:
         assert im.getexif() == {}
diff --git a/src/PIL/JpegXlImagePlugin.py b/src/PIL/JpegXlImagePlugin.py
index 62a6f4fa5..c4c2adac9 100644
--- a/src/PIL/JpegXlImagePlugin.py
+++ b/src/PIL/JpegXlImagePlugin.py
@@ -55,7 +55,8 @@ class JpegXlImageFile(ImageFile.ImageFile):
                 self.info["exif"] = exif[exif_start_offset + 4 :]
         if xmp := self._decoder.get_xmp():
             self.info["xmp"] = xmp
-        self.tile = [ImageFile._Tile("raw", (0, 0) + self.size, 0, self.mode)]
+        rawmode = "L" if self.mode == "1" else self.mode
+        self.tile = [ImageFile._Tile("raw", (0, 0) + self.size, 0, rawmode)]
 
     @property
     def n_frames(self) -> int:
diff --git a/src/_jpegxl.c b/src/_jpegxl.c
index e9c6284d4..a91a8c34e 100644
--- a/src/_jpegxl.c
+++ b/src/_jpegxl.c
@@ -14,21 +14,16 @@
 void
 _jxl_get_pixel_format(JxlPixelFormat *pf, const JxlBasicInfo *bi) {
     pf->num_channels = bi->num_color_channels + bi->num_extra_channels;
-
-    if (bi->exponent_bits_per_sample) {
-        pf->data_type = JXL_TYPE_FLOAT;
-    } else if (bi->bits_per_sample > 8) {
-        pf->data_type = JXL_TYPE_UINT16;
-    } else {
-        pf->data_type = JXL_TYPE_UINT8;
-    }
-
+    pf->data_type = bi->bits_per_sample > 8 ? JXL_TYPE_UINT16 : JXL_TYPE_UINT8;
     pf->align = 0;
 }
 
 char *
 _jxl_get_mode(const JxlBasicInfo *bi) {
     if (bi->num_color_channels == 1 && !bi->alpha_bits) {
+        if (bi->bits_per_sample == 1) {
+            return "1";
+        }
         if (bi->bits_per_sample == 16) {
             return "I;16";
         }
@@ -40,9 +35,6 @@ _jxl_get_mode(const JxlBasicInfo *bi) {
             if (bi->num_color_channels == 3) {
                 return bi->alpha_premultiplied ? "RGBa" : "RGBA";
             }
-            if (bi->num_color_channels == 1) {
-                return bi->alpha_premultiplied ? "La" : "LA";
-            }
         } else {
             // image has no transparency
             if (bi->num_color_channels == 3) {
@@ -290,7 +282,7 @@ decoder_loop_skip_process:
                     realloc(final_jxl_buf, final_jxl_buf_len + compressed_box_size);
                 if (!_new_jxl_buf) {
                     PyErr_SetString(PyExc_OSError, "failed to allocate final_jxl_buf");
-                    goto end;
+                    goto end_with_custom_error;
                 }
                 final_jxl_buf = _new_jxl_buf;
 
diff --git a/src/libImaging/Unpack.c b/src/libImaging/Unpack.c
index 203bcac2c..7456168b3 100644
--- a/src/libImaging/Unpack.c
+++ b/src/libImaging/Unpack.c
@@ -256,6 +256,14 @@ unpack18(UINT8 *out, const UINT8 *in, int pixels) {
     }
 }
 
+static void
+unpack1L(UINT8 *out, const UINT8 *in, int pixels) {
+    int i;
+    for (i = 0; i < pixels; i++) {
+        out[i] = in[i] > 128 ? 255 : 0;
+    }
+}
+
 /* Unpack to "L" image */
 
 static void
@@ -1564,6 +1572,7 @@ static struct {
     {IMAGING_MODE_1, IMAGING_RAWMODE_1_R, 1, unpack1R},
     {IMAGING_MODE_1, IMAGING_RAWMODE_1_IR, 1, unpack1IR},
     {IMAGING_MODE_1, IMAGING_RAWMODE_1_8, 8, unpack18},
+    {IMAGING_MODE_1, IMAGING_RAWMODE_L, 8, unpack1L},
 
     /* grayscale */
     {IMAGING_MODE_L, IMAGING_RAWMODE_L_2, 2, unpackL2},