diff --git a/ext/cbson/cbson.c b/ext/cbson/cbson.c
index 071fb9b..b16a873 100644
--- a/ext/cbson/cbson.c
+++ b/ext/cbson/cbson.c
@@ -1,4 +1,5 @@
 #include "ruby.h"
+#include "st.h"
 #include <assert.h>
 
 #define INITIAL_BUFFER_SIZE 256
@@ -9,6 +10,8 @@ typedef struct {
     int position;
 } bson_buffer;
 
+static char zero = 0;
+
 static bson_buffer* buffer_new(void) {
     bson_buffer* buffer;
     buffer = (bson_buffer*)malloc(sizeof(bson_buffer));
@@ -65,9 +68,51 @@ static void buffer_write_bytes(bson_buffer* buffer, const char* bytes, int size)
     buffer->position += size;
 }
 
+static void write_name_and_type(bson_buffer* buffer, VALUE name, char type) {
+    buffer_write_bytes(buffer, &type, 1);
+    buffer_write_bytes(buffer, RSTRING(name)->ptr, RSTRING(name)->len);
+    buffer_write_bytes(buffer, &zero, 1);
+}
+
+static int write_element(VALUE key, VALUE value, VALUE extra) {
+    bson_buffer* buffer = (bson_buffer*)extra;
+
+    switch(TYPE(value)) {
+    case T_STRING:
+        write_name_and_type(buffer, key, 0x02);
+        int length = RSTRING(value)->len + 1;
+        buffer_write_bytes(buffer, (char*)&length, 4);
+        buffer_write_bytes(buffer, RSTRING(value)->ptr, length - 1);
+        buffer_write_bytes(buffer, &zero, 1);
+        break;
+    default:
+        rb_raise(rb_eTypeError, "no c encoder for this type yet");
+        break;
+    }
+    return ST_CONTINUE;
+}
+
+static void write_doc(bson_buffer* buffer, VALUE hash) {
+    int start_position = buffer->position;
+    int length_location = buffer_save_bytes(buffer, 4);
+
+    rb_hash_foreach(hash, write_element, (VALUE)buffer);
+
+    // write null byte and fill in length
+    buffer_write_bytes(buffer, &zero, 1);
+    int length = buffer->position - start_position;
+    memcpy(buffer->buffer + length_location, &length, 4);
+}
+
 static VALUE method_serialize(VALUE self, VALUE doc) {
-    char data[5] = {0x05, 0x00, 0x00, 0x00, 0x00};
-    return rb_str_new(data, 5);
+    bson_buffer* buffer = buffer_new();
+    assert(buffer);
+
+    write_doc(buffer, doc);
+
+    VALUE result = rb_str_new(buffer->buffer, buffer->position);
+    buffer_free(buffer);
+    return result;
 }
 
 void Init_cbson() {
diff --git a/lib/mongo/util/bson.rb b/lib/mongo/util/bson.rb
index 0ea4c79..82bc835 100644
--- a/lib/mongo/util/bson.rb
+++ b/lib/mongo/util/bson.rb
@@ -255,7 +255,10 @@ class BSON
   def deserialize_string_data(buf)
     len = buf.get_int
     bytes = buf.get(len)
-    str = bytes[0..-2].pack("C*")
+    str = bytes[0..-2]
+    if str.respond_to? "pack"
+      str = str.pack("C*")
+    end
     if RUBY_VERSION >= '1.9'
       str.force_encoding("utf-8")
     end