diff --git a/src/calibre/devices/kindle/apnx.py b/src/calibre/devices/kindle/apnx.py
index 317a1287d1..89a673b25b 100644
--- a/src/calibre/devices/kindle/apnx.py
+++ b/src/calibre/devices/kindle/apnx.py
@@ -237,7 +237,7 @@ class APNXBuilder(object):
         # not modifying the text. In this case the case
         # doesn't matter just the absolute character and
         # the position within the stream.
-        data = bytearray(as_bytes(mr.mobi_html.lower()))
+        data = mr.mobi_html.lower()
         slash, p, lt, gt = map(ord, '/p<>')
         for c in data:
             pos += 1
diff --git a/src/calibre/devices/prs505/driver.py b/src/calibre/devices/prs505/driver.py
index f816754722..0bd4d7b015 100644
--- a/src/calibre/devices/prs505/driver.py
+++ b/src/calibre/devices/prs505/driver.py
@@ -135,10 +135,10 @@ class PRS505(USBMS):
                             time.sleep(5)
                             os.makedirs(dname, mode=0o777)
                     with lopen(cachep, 'wb') as f:
-                        f.write(u'''<?xml version="1.0" encoding="UTF-8"?>
+                        f.write(b'''<?xml version="1.0" encoding="UTF-8"?>
                             <cache xmlns="http://www.kinoma.com/FskCache/1">
                             </cache>
-                            '''.encode('utf8'))
+                            ''')
                         fsync(f)
                 return True
             except:
diff --git a/src/calibre/devices/prs505/sony_cache.py b/src/calibre/devices/prs505/sony_cache.py
index fb6f358137..cb4eca51d1 100644
--- a/src/calibre/devices/prs505/sony_cache.py
+++ b/src/calibre/devices/prs505/sony_cache.py
@@ -35,13 +35,13 @@ Periodical identifier sample from a PRS-650:
 '''
 
 # Utility functions {{{
-EMPTY_CARD_CACHE = '''\
+EMPTY_CARD_CACHE = b'''\
 <?xml version="1.0" encoding="UTF-8"?>
 <cache xmlns="http://www.kinoma.com/FskCache/1">
 </cache>
 '''
 
-EMPTY_EXT_CACHE = '''\
+EMPTY_EXT_CACHE = b'''\
 <?xml version="1.0" encoding="UTF-8"?>
 <cacheExt xmlns="http://www.sony.com/xmlns/product/prs/device/1">
 </cacheExt>
@@ -726,8 +726,8 @@ class XMLCache(object):
             self.cleanup_whitespace(i)
             raw = etree.tostring(self.roots[i], encoding='UTF-8',
                     xml_declaration=True)
-            raw = raw.replace("<?xml version='1.0' encoding='UTF-8'?>",
-                    '<?xml version="1.0" encoding="UTF-8"?>')
+            raw = raw.replace(b"<?xml version='1.0' encoding='UTF-8'?>",
+                    b'<?xml version="1.0" encoding="UTF-8"?>')
             with lopen(path, 'wb') as f:
                 f.write(raw)
                 fsync(f)
@@ -738,8 +738,8 @@ class XMLCache(object):
                     xml_declaration=True)
             except:
                 continue
-            raw = raw.replace("<?xml version='1.0' encoding='UTF-8'?>",
-                    '<?xml version="1.0" encoding="UTF-8"?>')
+            raw = raw.replace(b"<?xml version='1.0' encoding='UTF-8'?>",
+                    b'<?xml version="1.0" encoding="UTF-8"?>')
             with lopen(path, 'wb') as f:
                 f.write(raw)
                 fsync(f)
diff --git a/src/calibre/ebooks/__init__.py b/src/calibre/ebooks/__init__.py
index 8e958202f8..8028840733 100644
--- a/src/calibre/ebooks/__init__.py
+++ b/src/calibre/ebooks/__init__.py
@@ -204,7 +204,7 @@ def check_ebook_format(stream, current_guess):
     ans = current_guess
     if current_guess.lower() in ('prc', 'mobi', 'azw', 'azw1', 'azw3'):
         stream.seek(0)
-        if stream.read(3) == 'TPZ':
+        if stream.read(3) == b'TPZ':
             ans = 'tpz'
         stream.seek(0)
     return ans
diff --git a/src/calibre/ebooks/mobi/reader/mobi6.py b/src/calibre/ebooks/mobi/reader/mobi6.py
index bda6f97dc6..60e4f89aaf 100644
--- a/src/calibre/ebooks/mobi/reader/mobi6.py
+++ b/src/calibre/ebooks/mobi/reader/mobi6.py
@@ -346,7 +346,7 @@ class MobiReader(object):
     def cleanup_html(self):
         self.log.debug('Cleaning up HTML...')
         self.processed_html = re.sub(r'<div height="0(pt|px|ex|em|%){0,1}"></div>', '', self.processed_html)
-        if self.book_header.ancient and '<html' not in self.mobi_html[:300].lower():
+        if self.book_header.ancient and b'<html' not in self.mobi_html[:300].lower():
             self.processed_html = '<html><p>' + self.processed_html.replace('\n\n', '<p>') + '</html>'
         self.processed_html = self.processed_html.replace('\r\n', '\n')
         self.processed_html = self.processed_html.replace('> <', '>\n<')