本例子需要两个文件:xmlencode.htm和xmlencode.asp
======================================

xmlencode.htm
============
<script>
var xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
xmlhttp.open("GET","http://mengxianhui/asp/xmlencode.asp",false)
xmlhttp.setRequestHeader("Content-Type","text/xml")
xmlhttp.send()
alert(xmlhttp.responseXML.xml);
var s = '<?xml version="1.0"?><a> 如何解决XMLHTTP提交过时返回的是乱码，这是先提交上去，再返回的文字。 </a>'
xmlhttp.open("POST","http://mengxianhui/asp/xmlencode.asp",false)
xmlhttp.setRequestHeader("Content-Type","text/xml")
xmlhttp.send(s)
alert(xmlhttp.responseXML.xml);
</script>

xmlencode.asp
============
<%@ Language=VBScript %>
<%
Response.ContentType = "text/xml"
Response.CharSet = "GB2312"

Dim varCount,varReq,xmldom
varCount = Request.TotalBytes
varReq = Request.BinaryRead(varCount)

If Request.ServerVariables("REQUEST_METHOD") = "POST" Then
Set xmldom = Server.CreateObject("MSXML2.DOMDocument")
xmldom.load varReq
Response.Write "<?xml version=""1.0"" encoding=""GB2312""?>" & xmldom.documentElement.xml
Else
Response.Write "<?xml version=""1.0"" encoding=""GB2312""?><a>如何解决XMLHTTP提交过时返回的是乱码，这是从服务器直接返回的文字。</a>"
End If
%>

以上在中文WIN2K+IE6和英文WINXP+IE6和IE5下测试通过！