diff --git a/beets/library.py b/beets/library.py
index 7d02c32d1..f9285cabb 100644
--- a/beets/library.py
+++ b/beets/library.py
@@ -408,7 +408,7 @@ class Item(LibModel):
         except (OSError, IOError) as exc:
             raise ReadError(read_path, exc)
 
-        for key in list(self._media_fields):
+        for key in self._media_fields:
             value = getattr(mediafile, key)
             if isinstance(value, (int, long)):
                 # Filter values wider than 64 bits (in signed representation).
@@ -427,7 +427,8 @@ class Item(LibModel):
     def write(self, path=None):
         """Write the item's metadata to a media file.
 
-        Updates the mediafile with properties from itself.
+        All fields in `_media_fields` are written to disk according to
+        the values on this object.
 
         Can raise either a `ReadError` or a `WriteError`.
         """
@@ -442,8 +443,9 @@ class Item(LibModel):
 
         plugins.send('write', item=self, path=path)
 
+        mediafile.update(self)
         try:
-            mediafile.update(self, id3v23=beets.config['id3v23'].get(bool))
+            mediafile.save(id3v23=beets.config['id3v23'].get(bool))
         except (OSError, IOError, MutagenError) as exc:
             raise WriteError(self.path, exc)
 
diff --git a/beets/mediafile.py b/beets/mediafile.py
index c5e5efefa..c25fd3c86 100644
--- a/beets/mediafile.py
+++ b/beets/mediafile.py
@@ -1258,9 +1258,13 @@ class MediaFile(object):
             for tag in self.mgfile.keys():
                 del self.mgfile[tag]
 
+
+    # Convenient access to the set of available fields.
+
     @classmethod
     def fields(cls):
-        """Yield the names of all properties that are MediaFields.
+        """Get the names of all writable properties that reflect
+        metadata tags (i.e., those that are `MediaField`s).
         """
         for property, descriptor in cls.__dict__.items():
             if isinstance(descriptor, MediaField):
@@ -1268,13 +1272,13 @@ class MediaFile(object):
 
     @classmethod
     def readable_fields(cls):
-        """Yield the elements of ``fields()`` and all additional
-        properties retrieved from the file
+        """Get all metadata fields: the writable ones from `fields` and
+        also other audio properties.
         """
         for property in cls.fields():
             yield property
-        for property in ['length', 'samplerate', 'bitdepth', 'bitrate',
-                         'channels', 'format']:
+        for property in ('length', 'samplerate', 'bitdepth', 'bitrate',
+                         'channels', 'format'):
             yield property
 
     @classmethod
@@ -1282,9 +1286,7 @@ class MediaFile(object):
         """Add a field to store custom tags.
 
         ``name`` is the name of the property the field is accessed
-        through. It must not already exist for the class. If the name
-        coincides with the name of a property of ``Item`` it will be set
-        from the item in ``item.write()``.
+        through. It must not already exist on this class.
 
         ``descriptor`` must be an instance of ``MediaField``.
         """
@@ -1296,21 +1298,16 @@ class MediaFile(object):
                 u'property "{0}" already exists on MediaField'.format(name))
         setattr(cls, name, descriptor)
 
-    def update(self, dict, id3v23=False):
-        """Update tags from the dictionary and write them to the file.
+    def update(self, dict):
+        """Set all field values from a dictionary.
 
         For any key in ``dict`` that is also a field to store tags the
         method retrieves the corresponding value from ``dict`` and
-        updates the ``MediaFile``. The changes are then written to the
-        disk.
-
-        By default, MP3 files are saved with ID3v2.4 tags. You can use
-        the older ID3v2.3 standard by specifying the `id3v23` option.
+        updates the ``MediaFile``.
         """
         for field in self.fields():
             if field in dict:
                 setattr(self, field, dict[field])
-        self.save(id3v23)
 
 
     # Field definitions.
diff --git a/test/test_mediafile.py b/test/test_mediafile.py
index 6f86f6c88..5548e364a 100644
--- a/test/test_mediafile.py
+++ b/test/test_mediafile.py
@@ -194,6 +194,7 @@ class LazySaveTestMixin(object):
         self.assertEqual(os.stat(mediafile.path).st_mtime, mtime)
 
         mediafile.update({'title': mediafile.title})
+        mediafile.save()
         self.assertEqual(os.stat(mediafile.path).st_mtime, mtime)
 
     @unittest.skip('not yet implemented')
@@ -213,6 +214,7 @@ class LazySaveTestMixin(object):
         self.assertEqual(os.stat(mediafile.path).st_mtime, mtime)
 
         mediafile.update({'title': mediafile.title, 'album': 'another'})
+        mediafile.save()
         self.assertNotEqual(os.stat(mediafile.path).st_mtime, mtime)
 
     def _set_past_mtime(self, path):
@@ -297,6 +299,7 @@ class ExtendedFieldTestMixin(object):
 
         mediafile = self._mediafile_fixture('empty')
         mediafile.update({'initialkey': 'F#'})
+        mediafile.save()
 
         item = Item.from_path(mediafile.path)
         self.assertEqual(item['initialkey'], 'F#')
@@ -441,6 +444,7 @@ class ReadWriteTestBase(ArtTestMixin, GenreListTestMixin,
         tags = self._generate_tags()
 
         mediafile.update(tags)
+        mediafile.save()
 
         mediafile = MediaFile(mediafile.path)
         self.assertTags(mediafile, tags)
@@ -466,8 +470,10 @@ class ReadWriteTestBase(ArtTestMixin, GenreListTestMixin,
         tags = self._generate_tags()
 
         mediafile.update(tags)
+        mediafile.save()
         # Make sure the tags are already set when writing a second time
         mediafile.update(tags)
+        mediafile.save()
 
         mediafile = MediaFile(mediafile.path)
         self.assertTags(mediafile, tags)