<script language="JavaScript"> 
<!-- 

var xmlhttp; 

function ajax() { 

   if (window.XMLHttpRequest) { 
      xmlhttp = new XMLHttpRequest(); 
   } else if (window.ActiveXObject) { 
      xmlhttp = new ActiveXObject("Microsoft.XMLHTTP"); 
   } else { 
       alert("Seu navegador n&atilde;o suporta XMLHttpRequest."); 
      return; 
   } 

    xmlhttp.open("GET", "teste.php?texto=" + url_encode("teste... éé"), true); 
    xmlhttp.onreadystatechange = processReqChange; 
    xmlhttp.send(null); 
} 

function processReqChange() { 
    if (xmlhttp.readyState == 4) { 
       if (xmlhttp.status == 200) { 
           document.getElementById("texto").innerHTML = url_decode(xmlhttp.responseText); 
      } else { 
          alert("Problemas ao carregar o arquivo."); 
      } 
    } 
} 


// url_encode version 1.0 
function url_encode(str) { 
    var hex_chars = "0123456789ABCDEF"; 
    var noEncode = /^([a-zA-Z0-9\_\-\.])$/; 
    var n, strCode, hex1, hex2, strEncode = ""; 

    for(n = 0; n < str.length; n++) { 
        if (noEncode.test(str.charAt(n))) { 
            strEncode += str.charAt(n); 
        } else { 
            strCode = str.charCodeAt(n); 
            hex1 = hex_chars.charAt(Math.floor(strCode / 16)); 
            hex2 = hex_chars.charAt(strCode % 16); 
            strEncode += "%" + (hex1 + hex2); 
        } 
    } 
    return strEncode; 
} 

// url_decode version 1.0 
function url_decode(str) { 
    var n, strCode, strDecode = ""; 

    for (n = 0; n < str.length; n++) { 
        if (str.charAt(n) == "%") { 
            strCode = str.charAt(n + 1) + str.charAt(n + 2); 
            strDecode += String.fromCharCode(parseInt(strCode, 16)); 
            n += 2; 
        } else { 
            strDecode += str.charAt(n); 
        } 
    } 
    return strDecode; 
} 
//--> 
</script> 
<a href="javascript:ajax()">Testar</a> 
<div id="texto"></div>
E a página teste.php que estou chamando pelo ajax. 
<?php 
$texto = rawurldecode($_GET["texto"]); 
echo "O texto '$texto' não teve problemas na acentuação."; 
?>