diff --git a/beets/library.py b/beets/library.py
index 789b4c72f..d040e1c18 100644
--- a/beets/library.py
+++ b/beets/library.py
@@ -186,7 +186,7 @@ ITEM_FIELDS = [
 ]
 ITEM_KEYS          = [f[0] for f in ITEM_FIELDS]
 ITEM_KEYS_WRITABLE = set(MediaFile.fields()).intersection(ITEM_KEYS)
-ITEM_KEYS_META     = set(MediaFile.readable_fields()).intersection(ITEM_KEYS)
+
 
 # Database fields for the "albums" table.
 # The third entry in each tuple indicates whether the field reflects an
@@ -333,6 +333,13 @@ class Item(LibModel):
     _flex_table = 'item_attributes'
     _search_fields = ITEM_DEFAULT_FIELDS
 
+    media_fields = set(MediaFile.readable_fields()).intersection(ITEM_KEYS)
+    """Set of property names to read from ``MediaFile``.
+
+    ``item.read()`` will read all properties in this set from
+    ``MediaFile`` and set them on the item.
+    """
+
     @classmethod
     def _getters(cls):
         return plugins.item_field_getters()
@@ -383,8 +390,11 @@ class Item(LibModel):
     # Interaction with file metadata.
 
     def read(self, read_path=None):
-        """Read the metadata from the associated file. If read_path is
-        specified, read metadata from that file instead.
+        """Read the metadata from the associated file.
+
+        If ``read_path`` is specified, read metadata from that file
+        instead. Updates all the properties in ``Item.media_fields``
+        from the media file.
 
         Raises a `ReadError` if the file could not be read.
         """
@@ -393,20 +403,19 @@ class Item(LibModel):
         else:
             read_path = normpath(read_path)
         try:
-            f = MediaFile(syspath(read_path))
+            mediafile = MediaFile(syspath(read_path))
         except (OSError, IOError) as exc:
             raise ReadError(read_path, exc)
 
-        for key in list(ITEM_KEYS_META) + MediaFile.custom_fields:
-            value = getattr(f, key)
+        for key in list(self.media_fields):
+            value = getattr(mediafile, key)
             if isinstance(value, (int, long)):
-                # Filter values wider than 64 bits (in signed
-                # representation). SQLite cannot store them.
-                # py26: Post transition, we can use:
+                # Filter values wider than 64 bits (in signed representation).
+                # SQLite cannot store them. py26: Post transition, we can use:
                 # value.bit_length() > 63
                 if abs(value) >= 2 ** 63:
                     value = 0
-            setattr(self, key, value)
+            self[key] = value
 
         # Database's mtime should now reflect the on-disk value.
         if read_path == self.path:
@@ -417,6 +426,8 @@ class Item(LibModel):
     def write(self):
         """Write the item's metadata to the associated file.
 
+        Updates the mediafile with properties from itself.
+
         Can raise either a `ReadError` or a `WriteError`.
         """
         try:
diff --git a/beets/mediafile.py b/beets/mediafile.py
index 7c47e54f2..3f207ef20 100644
--- a/beets/mediafile.py
+++ b/beets/mediafile.py
@@ -1276,8 +1276,6 @@ class MediaFile(object):
                          'channels', 'format']:
             yield property
 
-    custom_fields = []
-
     @classmethod
     def add_field(cls, name, descriptor):
         """Add a field to store custom tags.
@@ -1295,7 +1293,6 @@ class MediaFile(object):
         if name in cls.__dict__:
             raise ValueError(
                 u'property "{0}" already exists on MediaField'.format(name))
-        cls.custom_fields.append(name)
         setattr(cls, name, descriptor)
 
     def update(self, dict, id3v23=False):
diff --git a/beets/ui/commands.py b/beets/ui/commands.py
index d45192be1..024e3a3b7 100644
--- a/beets/ui/commands.py
+++ b/beets/ui/commands.py
@@ -923,7 +923,7 @@ def update_items(lib, query, album, move, pretend):
 
             # Check for and display changes.
             changed = ui.show_model_changes(item,
-                                            fields=library.ITEM_KEYS_META)
+                                            fields=library.Item.media_fields)
 
             # Save changes.
             if not pretend:
diff --git a/test/test_mediafile.py b/test/test_mediafile.py
index 4334f3c7e..4f8a1b14c 100644
--- a/test/test_mediafile.py
+++ b/test/test_mediafile.py
@@ -272,9 +272,8 @@ class ExtendedFieldTestMixin(object):
         mediafile = MediaFile(mediafile.path)
         self.assertEqual(mediafile.initialkey, 'F#')
         delattr(MediaFile, 'initialkey')
-        MediaFile.custom_fields = []
 
-    def test_extended_attribute_from_item(self):
+    def test_write_extended_tag_from_item(self):
         MediaFile.add_field('initialkey', field_extension)
 
         mediafile = self._mediafile_fixture('empty')
@@ -284,11 +283,12 @@ class ExtendedFieldTestMixin(object):
         item.write()
         mediafile = MediaFile(mediafile.path)
         self.assertEqual(mediafile.initialkey, 'Gb')
-        delattr(MediaFile, 'initialkey')
-        MediaFile.custom_fields = []
 
-    def test_flexible_attribute_from_file(self):
+        delattr(MediaFile, 'initialkey')
+
+    def test_read_flexible_attribute_from_file(self):
         MediaFile.add_field('initialkey', field_extension)
+        Item.media_fields.add('initialkey')
 
         mediafile = self._mediafile_fixture('empty')
         mediafile.update({'initialkey': 'F#'})
@@ -297,7 +297,7 @@ class ExtendedFieldTestMixin(object):
         self.assertEqual(item['initialkey'], 'F#')
 
         delattr(MediaFile, 'initialkey')
-        MediaFile.custom_fields = []
+        Item.media_fields.remove('initialkey')
 
     def test_invalid_descriptor(self):
         with self.assertRaises(ValueError) as cm: