JSON result of .ajax() is UTF8 but not always seen as UTF8 ?!?

JSON result of .ajax() is UTF8 but not always seen as UTF8 ?!?

Hello,
I have an ajax function that receives form data serialized and returns a JSON object. The odd thing is that sometimes the answer is correctly seen as UTF-8 but most of the times, it is seen as something else (latin1 ?) and I don't see "Ce nom est déjà pris" but "Ce nom est déjà pris" (UTF-8 re-encoding).

The ajax function is called like this :
  1.         $.ajax({
  2.             url: "/ajax/validateUserForm.html",
  3.             dataType: "json",
  4.             data: $("form#userform").serialize(),
  5.             contentType: "charset=utf-8",// useless
  6.             type: "POST"
  7.         }).done( function(result) {
  8.            
  9.              $.each(result, function(key, val) {
  10.                  console.info(key+' : '+val);
  11.              });
  12.         });
Server side, I checked the result sent back is (always) UTF8.
Firebug tells me the response always has :
  1. Content-Type: text/html; charset=utf-8
Any clue ?

Thanks for your help.