diff --git a/src/calibre/devices/mtp/unix/libmtp.c b/src/calibre/devices/mtp/unix/libmtp.c
index 8d8481d551..eb99375136 100644
--- a/src/calibre/devices/mtp/unix/libmtp.c
+++ b/src/calibre/devices/mtp/unix/libmtp.c
@@ -86,7 +86,11 @@ static uint16_t data_to_python(void *params, void *priv, uint32_t sendlen, unsig
     cb = (ProgressCallback *)priv;
     *putlen = sendlen;
     PyEval_RestoreThread(cb->state);
+#if PY_MAJOR_VERSION >= 3
+    res = PyObject_CallMethod(cb->extra, "write", "y#", data, (Py_ssize_t)sendlen);
+#else
     res = PyObject_CallMethod(cb->extra, "write", "s#", data, (Py_ssize_t)sendlen);
+#endif
     if (res == NULL) {
         ret = LIBMTP_HANDLER_RETURN_ERROR;
         *putlen = 0;
diff --git a/src/calibre/devices/mtp/windows/content_enumeration.cpp b/src/calibre/devices/mtp/windows/content_enumeration.cpp
index 67fe2b7914..9afff2461d 100644
--- a/src/calibre/devices/mtp/windows/content_enumeration.cpp
+++ b/src/calibre/devices/mtp/windows/content_enumeration.cpp
@@ -554,7 +554,11 @@ PyObject* wpd::get_file(IPortableDevice *device, const wchar_t *object_id, PyObj
             PyErr_SetString(PyExc_IOError, "Read access is denied to this object"); break;
         } else if (SUCCEEDED(hr)) {
             if (bytes_read > 0) {
+#if PY_MAJOR_VERSION >= 3
+                res = PyObject_CallMethod(dest, "write", "y#", buf, bytes_read);
+#else
                 res = PyObject_CallMethod(dest, "write", "s#", buf, bytes_read);
+#endif
                 if (res == NULL) break;
                 Py_DECREF(res); res = NULL;
                 if (callback != NULL) Py_XDECREF(PyObject_CallFunction(callback, "kK", total_read, filesize));
diff --git a/src/calibre/ebooks/compression/palmdoc.c b/src/calibre/ebooks/compression/palmdoc.c
index 61d00ab04f..623bb10f67 100644
--- a/src/calibre/ebooks/compression/palmdoc.c
+++ b/src/calibre/ebooks/compression/palmdoc.c
@@ -42,12 +42,20 @@ typedef struct {
 
 #define CHAR(x) (( (x) > 127 ) ? (x)-256 : (x))
 
+#if PY_MAJOR_VERSION >= 3
+    #define BUFFER_FMT "y*"
+    #define BYTES_FMT "y#"
+#else
+    #define BUFFER_FMT "t#"
+    #define BYTES_FMT "s#"
+#endif
+
 static PyObject *
 cpalmdoc_decompress(PyObject *self, PyObject *args) {
     const char *_input = NULL; Py_ssize_t input_len = 0;
     Byte *input; char *output; Byte c; PyObject *ans;
     Py_ssize_t i = 0, o = 0, j = 0, di, n;
-    if (!PyArg_ParseTuple(args, "t#", &_input, &input_len))
+    if (!PyArg_ParseTuple(args, BUFFER_FMT, &_input, &input_len))
 		return NULL;
     input = (Byte *) PyMem_Malloc(sizeof(Byte)*input_len);
     if (input == NULL) return PyErr_NoMemory();
@@ -76,7 +84,7 @@ cpalmdoc_decompress(PyObject *self, PyObject *args) {
                 output[o] = output[o - di];
         }
     }
-    ans = Py_BuildValue("s#", output, o);
+    ans = Py_BuildValue(BYTES_FMT, output, o);
     if (output != NULL) PyMem_Free(output);
     if (input != NULL) PyMem_Free(input);
     return ans;
@@ -162,7 +170,7 @@ cpalmdoc_compress(PyObject *self, PyObject *args) {
     char *output; PyObject *ans;
     Py_ssize_t j = 0;
     buffer b;
-    if (!PyArg_ParseTuple(args, "t#", &_input, &input_len))
+    if (!PyArg_ParseTuple(args, BUFFER_FMT, &_input, &input_len))
 		return NULL;
     b.data = (Byte *)PyMem_Malloc(sizeof(Byte)*input_len);
     if (b.data == NULL) return PyErr_NoMemory();
@@ -176,7 +184,7 @@ cpalmdoc_compress(PyObject *self, PyObject *args) {
     if (output == NULL) return PyErr_NoMemory();
     j = cpalmdoc_do_compress(&b, output);
     if ( j == 0) return PyErr_NoMemory();
-    ans = Py_BuildValue("s#", output, j);
+    ans = Py_BuildValue(BYTES_FMT, output, j);
     PyMem_Free(output);
     PyMem_Free(b.data);
     return ans;
@@ -203,4 +211,3 @@ initcPalmdoc(void) {
     );
     if (m == NULL) return;
 }
-
diff --git a/src/calibre/utils/fonts/winfonts.cpp b/src/calibre/utils/fonts/winfonts.cpp
index 428f31d529..bafc8b5728 100644
--- a/src/calibre/utils/fonts/winfonts.cpp
+++ b/src/calibre/utils/fonts/winfonts.cpp
@@ -165,7 +165,11 @@ static PyObject* add_font(PyObject *self, PyObject *args) {
     Py_ssize_t sz;
     DWORD num = 0;
 
+#if PY_MAJOR_VERSION >= 3
+    if (!PyArg_ParseTuple(args, "y#", &data, &sz)) return NULL;
+#else
     if (!PyArg_ParseTuple(args, "s#", &data, &sz)) return NULL;
+#endif
 
     AddFontMemResourceEx(data, (DWORD)sz, NULL, &num);
 
@@ -260,4 +264,3 @@ initwinfonts(void) {
     PyModule_AddIntMacro(m, FW_HEAVY);
     PyModule_AddIntMacro(m, FW_BLACK);
 }
-
diff --git a/src/calibre/utils/lzx/compressor.c b/src/calibre/utils/lzx/compressor.c
index b63ee3ee95..605ca515f5 100644
--- a/src/calibre/utils/lzx/compressor.c
+++ b/src/calibre/utils/lzx/compressor.c
@@ -310,7 +310,11 @@ Compressor_compress(Compressor *self, PyObject *args, PyObject *kwds)
     int flush = 0;
 
     if (!PyArg_ParseTupleAndKeywords(
+#if PYTHON_MAJOR_VERSION >= 3
+            args, kwds, "y#|b", kwlist, &data, &inlen, &flush)) {
+#else
             args, kwds, "s#|b", kwlist, &data, &inlen, &flush)) {
+#endif
         return NULL;
     }
 
diff --git a/src/calibre/utils/lzx/lzxmodule.c b/src/calibre/utils/lzx/lzxmodule.c
index 6c9a4f15d8..3cb5d36919 100644
--- a/src/calibre/utils/lzx/lzxmodule.c
+++ b/src/calibre/utils/lzx/lzxmodule.c
@@ -157,7 +157,11 @@ decompress(PyObject *self, PyObject *args)
     memory_file dest;
     PyObject *retval = NULL;
 
+#if PY_MAJOR_VERSION >= 3
+    if (!PyArg_ParseTuple(args, "y#I", &inbuf, &inlen, &outlen)) {
+#else
     if (!PyArg_ParseTuple(args, "s#I", &inbuf, &inlen, &outlen)) {
+#endif
         return NULL;
     }
 
diff --git a/src/calibre/utils/podofo/doc.cpp b/src/calibre/utils/podofo/doc.cpp
index 8c54782205..650ed3d149 100644
--- a/src/calibre/utils/podofo/doc.cpp
+++ b/src/calibre/utils/podofo/doc.cpp
@@ -32,12 +32,18 @@ PDFDoc_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 }
 // }}}
 
+#if PY_MAJOR_VERSION >= 3
+    #define BYTES_FMT "y#"
+#else
+    #define BYTES_FMT "s#"
+#endif
+
 // Loading/Opening of PDF files {{{
 static PyObject *
 PDFDoc_load(PDFDoc *self, PyObject *args) {
     char *buffer; Py_ssize_t size;
 
-    if (PyArg_ParseTuple(args, "s#", &buffer, &size)) {
+    if (PyArg_ParseTuple(args, BYTES_FMT, &buffer, &size)) {
         try {
 #if PODOFO_VERSION <= 0x000905
             self->doc->Load(buffer, (long)size);
@@ -77,7 +83,7 @@ static PyObject *
 PDFDoc_save(PDFDoc *self, PyObject *args) {
     char *buffer;
 
-    if (PyArg_ParseTuple(args, "s", &buffer)) {
+    if (PyArg_ParseTuple(args, BYTES_FMT, &buffer)) {
         try {
             self->doc->Write(buffer);
         } catch(const PdfError & err) {
@@ -284,7 +290,7 @@ PDFDoc_get_xmp_metadata(PDFDoc *self, PyObject *args) {
             if ((str = metadata->GetStream()) != NULL) {
                 str->GetFilteredCopy(&buf, &len);
                 if (buf != NULL) {
-                    ans = Py_BuildValue("s#", buf, len);
+                    ans = Py_BuildValue(BYTES_FMT, buf, len);
                     free(buf); buf = NULL;
                     if (ans == NULL) goto error;
                 }
@@ -312,7 +318,7 @@ PDFDoc_set_xmp_metadata(PDFDoc *self, PyObject *args) {
     TVecFilters compressed(1);
     compressed[0] = ePdfFilter_FlateDecode;
 
-    if (!PyArg_ParseTuple(args, "s#", &raw, &len)) return NULL;
+    if (!PyArg_ParseTuple(args, BYTES_FMT, &raw, &len)) return NULL;
     try {
         if ((metadata = self->doc->GetMetadata()) != NULL) {
             if ((str = metadata->GetStream()) == NULL) { PyErr_NoMemory(); goto error; }
diff --git a/src/lzma/lzma_binding.c b/src/lzma/lzma_binding.c
index 24dd28e60a..d02f14c523 100644
--- a/src/lzma/lzma_binding.c
+++ b/src/lzma/lzma_binding.c
@@ -58,6 +58,12 @@ static PyObject *LZMAError = NULL;
 // Utils {{{
 static UInt64 crc64_table[256];
 
+#if PY_MAJOR_VERSION >= 3
+    #define BYTES_FMT "y#"
+#else
+    #define BYTES_FMT "s#"
+#endif
+
 static void init_crc_table() {
     static const UInt64 poly64 = (UInt64)(0xC96C5795D7870F42);
     size_t i, j;
@@ -79,7 +85,7 @@ crc64(PyObject *self, PyObject *args) {
     Py_ssize_t size = 0;
     UInt64 crc = 0;
     size_t i;
-    if (!PyArg_ParseTuple(args, "s#|K", &data, &size, &crc)) return NULL;
+    if (!PyArg_ParseTuple(args, BYTES_FMT "|K", &data, &size, &crc)) return NULL;
     crc = ~crc;
     for (i = 0; i < (size_t)size; ++i)
         crc = crc64_table[data[i] ^ (crc & 0xFF)] ^ (crc >> 8);
@@ -142,7 +148,7 @@ decompress2(PyObject *self, PyObject *args) {
         } else { res = SZ_OK; bytes_written = 0; status = LZMA_STATUS_NEEDS_MORE_INPUT; }
         if (res != SZ_OK) { SET_ERROR(res); goto exit; }
         if (bytes_written > 0) {
-            if(!PyObject_CallFunction(write, "s#", outbuf, bytes_written)) goto exit;
+            if(!PyObject_CallFunction(write, BYTES_FMT, outbuf, bytes_written)) goto exit;
         }
         if (inbuf_len > inbuf_pos && !bytes_read && !bytes_written && status != LZMA_STATUS_NEEDS_MORE_INPUT && status != LZMA_STATUS_FINISHED_WITH_MARK) {
             SET_ERROR(SZ_ERROR_DATA); goto exit;
@@ -188,7 +194,7 @@ decompress(PyObject *self, PyObject *args) {
     ELzmaStatus status = LZMA_STATUS_NOT_FINISHED;
     ELzmaFinishMode finish_mode = LZMA_FINISH_ANY;
 
-    if(!PyArg_ParseTuple(args, "OOOKs#k", &read, &seek, &write, &decompressed_size, &header, &header_size, &bufsize)) return NULL;
+    if(!PyArg_ParseTuple(args, "OOOK" BYTES_FMT "k", &read, &seek, &write, &decompressed_size, &header, &header_size, &bufsize)) return NULL;
     size_known = (decompressed_size != (UInt64)(Int64)-1);
     if (header_size != 13) { PyErr_SetString(LZMAError, "Header must be exactly 13 bytes long"); return NULL; }
     if (!decompressed_size) { PyErr_SetString(LZMAError, "Cannot decompress empty file"); return NULL; }
@@ -214,7 +220,7 @@ decompress(PyObject *self, PyObject *args) {
         } else { res = SZ_OK; bytes_written = 0; status = LZMA_STATUS_NEEDS_MORE_INPUT; }
         if (res != SZ_OK) { SET_ERROR(res); goto exit; }
         if (bytes_written > 0) {
-            if(!PyObject_CallFunction(write, "s#", outbuf, bytes_written)) goto exit;
+            if(!PyObject_CallFunction(write, BYTES_FMT, outbuf, bytes_written)) goto exit;
             total_written += bytes_written;
         }
         if (inbuf_len > inbuf_pos && !bytes_read && !bytes_written && status != LZMA_STATUS_NEEDS_MORE_INPUT && status != LZMA_STATUS_FINISHED_WITH_MARK) {
@@ -296,7 +302,7 @@ static size_t owrite(void *p, const void *buf, size_t size) {
     PyObject *res = NULL;
     if (!size) return 0;
     ACQUIRE_GIL
-    res = PyObject_CallFunction(self->write, "s#", (char*)buf, size);
+    res = PyObject_CallFunction(self->write, BYTES_FMT, (char*)buf, size);
     if (res == NULL) return 0;
     Py_DECREF(res);
     RELEASE_GIL
@@ -332,7 +338,7 @@ get_lzma2_properties(int preset) {
 exit:
     if (lzma2) Lzma2Enc_Destroy(lzma2);
     if (PyErr_Occurred()) return NULL;
-    return Py_BuildValue("s#", &props_out, 1);
+    return Py_BuildValue(BYTES_FMT, &props_out, 1);
 }
 
 
@@ -382,7 +388,7 @@ compress(PyObject *self, PyObject *args) {
 exit:
     if (lzma2) Lzma2Enc_Destroy(lzma2);
     if (PyErr_Occurred()) return NULL;
-    return Py_BuildValue("s#", &props_out, 1);
+    return Py_BuildValue(BYTES_FMT, &props_out, 1);
 }
 
 // }}}