diff --git a/src/calibre/ebooks/pdb/palmdoc/reader.py b/src/calibre/ebooks/pdb/palmdoc/reader.py
index 67c4b2518e..6c41588246 100644
--- a/src/calibre/ebooks/pdb/palmdoc/reader.py
+++ b/src/calibre/ebooks/pdb/palmdoc/reader.py
@@ -8,9 +8,8 @@ __license__   = 'GPL v3'
 __copyright__ = '2009, John Schember <john@nachtimwald.com>'
 __docformat__ = 'restructuredtext en'
 
-import struct
+import struct, io
 
-from cStringIO import StringIO
 
 from calibre.ebooks.pdb.formatreader import FormatReader
 
@@ -53,7 +52,7 @@ class Reader(FormatReader):
         return ''
 
     def extract_content(self, output_dir):
-        raw_txt = ''
+        raw_txt = b''
 
         self.log.info('Decompressing text...')
         for i in range(1, self.header_record.num_records + 1):
@@ -61,7 +60,7 @@ class Reader(FormatReader):
             raw_txt += self.decompress_text(i)
 
         self.log.info('Converting text to OEB...')
-        stream = StringIO(raw_txt)
+        stream = io.BytesIO(raw_txt)
 
         from calibre.customize.ui import plugin_for_input_format
 
diff --git a/src/calibre/ebooks/pdb/ztxt/reader.py b/src/calibre/ebooks/pdb/ztxt/reader.py
index ebdd7ead94..8bfda3f59a 100644
--- a/src/calibre/ebooks/pdb/ztxt/reader.py
+++ b/src/calibre/ebooks/pdb/ztxt/reader.py
@@ -10,8 +10,8 @@ __docformat__ = 'restructuredtext en'
 
 import struct
 import zlib
+import io
 
-from cStringIO import StringIO
 
 from calibre.ebooks.pdb.formatreader import FormatReader
 from calibre.ebooks.pdb.ztxt import zTXTError
@@ -72,7 +72,7 @@ class Reader(FormatReader):
         return self.uncompressor.decompress(self.section_data(number))
 
     def extract_content(self, output_dir):
-        raw_txt = ''
+        raw_txt = b''
 
         self.log.info('Decompressing text...')
         for i in range(1, self.header_record.num_records + 1):
@@ -80,7 +80,7 @@ class Reader(FormatReader):
             raw_txt += self.decompress_text(i)
 
         self.log.info('Converting text to OEB...')
-        stream = StringIO(raw_txt)
+        stream = io.BytesIO(raw_txt)
 
         from calibre.customize.ui import plugin_for_input_format