diff --git a/Lib/xml/etree/ElementTree.py b/Lib/xml/etree/ElementTree.py
index c26a764..b404e20 100644
--- a/Lib/xml/etree/ElementTree.py
+++ b/Lib/xml/etree/ElementTree.py
@@ -96,6 +96,7 @@ VERSION = "1.3.0"
 # structure, and convert it from and to XML.
 ##
 
+import io
 import sys
 import re
 import warnings
@@ -811,43 +812,64 @@ class ElementTree:
         elif method not in _serialize:
             # FIXME: raise an ImportError for c14n if ElementC14N is missing?
             raise ValueError("unknown method %r" % method)
-        if hasattr(file_or_filename, "write"):
-            file = file_or_filename
+
+        if encoding:
+            _encoding = encoding
         else:
+            import locale
+            _encoding = locale.getpreferredencoding()
+
+        close_required = False
+        bytes_write = False
+        if isinstance(file_or_filename, io.BufferedIOBase):
+            file = file_or_filename
+            bytes_write = True
+        elif isinstance(file_or_filename, io.TextIOBase):
+            bytes_write = False
+            try:
+                fd = file_or_filename.fileno()
+            except io.UnsupportedOperation:
+                fd = -1
+            if fd >= 0:
+                if file_or_filename.encoding:
+                    _encoding = file_or_filename.encoding
+                file = open(fd, "w",
+                            encoding=_encoding, errors="xmlcharrefreplace",
+                            closefd=False)
+            else:
+                file = file_or_filename
+        elif hasattr(file_or_filename, "write"):
+            file = file_or_filename
             if encoding:
-                file = open(file_or_filename, "wb")
+                bytes_write = True
             else:
-                file = open(file_or_filename, "w")
-        if encoding:
+                bytes_write = False
+        else:
+            file = open(file_or_filename, "w",
+                        encoding=_encoding, errors="xmlcharrefreplace")
+            bytes_write = False
+            close_required = True
+        if bytes_write:
             def write(text):
                 try:
-                    return file.write(text.encode(encoding,
+                    return file.write(text.encode(_encoding,
                                                   "xmlcharrefreplace"))
                 except (TypeError, AttributeError):
                     _raise_serialization_error(text)
         else:
             write = file.write
-        if not encoding:
-            if method == "c14n":
-                encoding = "utf-8"
-            else:
-                encoding = None
-        elif xml_declaration or (xml_declaration is None and
-                                 encoding not in ("utf-8", "us-ascii")):
+
+        if xml_declaration or _encoding.upper() not in ("UTF-8", "US-ASCII"):
             if method == "xml":
-                encoding_ = encoding
-                if not encoding:
-                    # Retrieve the default encoding for the xml declaration
-                    import locale
-                    encoding_ = locale.getpreferredencoding()
-                write("<?xml version='1.0' encoding='%s'?>\n" % encoding_)
+                write("<?xml version='1.0' encoding='%s'?>\n" % _encoding)
+
         if method == "text":
             _serialize_text(write, self._root)
         else:
             qnames, namespaces = _namespaces(self._root, default_namespace)
             serialize = _serialize[method]
             serialize(write, self._root, qnames, namespaces)
-        if file_or_filename is not file:
+        if close_required:
             file.close()
 
     def write_c14n(self, file):