diff --git a/ajson.hpp b/ajson.hpp
index 036f6ba..02bc55b 100644
--- a/ajson.hpp
+++ b/ajson.hpp
@@ -1037,23 +1037,23 @@ namespace ajson
               }
               else
               {
-                unsigned char c1 = (uint8_t)(codepoint >> 24);
-                unsigned char c2 = (uint8_t)(codepoint >> 16);
-                unsigned char c3 = (uint8_t)(codepoint >> 8);
-                unsigned char c4 = (uint8_t)codepoint;
+                // utf-16 surrogate pair encoding (\uXXXX\uYYYY)
+                uint32_t cp_prime = codepoint - 0x10000;
+                uint16_t high = 0xD800 + (cp_prime >> 10);
+                uint16_t low  = 0xDC00 + (cp_prime & 0x3FF);
 
                 put('\\');
                 put('u');
-                put(hex_table[(c1) >> 4]);
-                put(hex_table[(c1)& 0xF]);
-                put(hex_table[(c2) >> 4]);
-                put(hex_table[(c2)& 0xF]);
+                put(hex_table[(high >> 12) & 0xF]);
+                put(hex_table[(high >> 8) & 0xF]);
+                put(hex_table[(high >> 4) & 0xF]);
+                put(hex_table[high & 0xF]);
                 put('\\');
                 put('u');
-                put(hex_table[(c3) >> 4]);
-                put(hex_table[(c3)& 0xF]);
-                put(hex_table[(c4) >> 4]);
-                put(hex_table[(c4)& 0xF]);
+                put(hex_table[(low >> 12) & 0xF]);
+                put(hex_table[(low >> 8) & 0xF]);
+                put(hex_table[(low >> 4) & 0xF]);
+                put(hex_table[low & 0xF]);
               }
             }
             else
@@ -1452,7 +1452,7 @@ namespace ajson
       str.append(1, (char)(0xC0 | ((utf1 >> 6) & 0xFF)));
       str.append(1, (char)(0x80 | ((utf1 & 0x3F))));
     }
-    else if (utf1 < 0x80000)
+    else if (utf1 < 0x10000)
     {
       str.append(1, (char)(0xE0 | ((utf1 >> 12) & 0xFF)));
       str.append(1, (char)(0x80 | ((utf1 >> 6) & 0x3F)));
@@ -1460,7 +1460,7 @@ namespace ajson
     }
     else
     {
-      if (utf1 < 0x110000)
+      if (utf1 >= 0x110000)
       {
         return false;
       }
@@ -1475,6 +1475,7 @@ namespace ajson
   template<typename string_ty>
   bool escape_string(string_ty& str , const char * data , size_t len)
   {
+    uint64_t high = 0;
     str.clear();
     str.reserve(len);
     if (len == 0)
@@ -1539,6 +1540,23 @@ namespace ajson
           len -= 4;
           if (uft1 == 0)
             return false;
+          if (uft1 >= 0xD800 && uft1 <= 0xDBFF) // high surrogate
+          {
+            if (high)
+              return false; // already have a high surrogate, error
+            high = uft1;
+            continue;
+          }
+          else if (uft1 >= 0xDC00 && uft1 <= 0xDFFF) // low surrogate
+          {
+            if (!high)
+              return false; // no preceding high surrogate, error
+            else
+            {
+              uft1 = 0x10000 + ((high - 0xD800) << 10) + (uft1 - 0xDC00); // claculate code point
+              high = 0;
+            }
+          }
           if (!esacpe_utf8(str, uft1))
             return false;
           continue;
@@ -1553,9 +1571,11 @@ namespace ajson
       }
       str.append(1, c);
     } while (len > 0);
+    if (high)
+      return false; // high surrogate without valid low surrogate
     return true;
   }
-  
+
   template<typename ty>
   struct json_impl < ty,
     typename std::enable_if <detail::is_stdstring<ty>::value>::type >