The API only returns data in JSON format - you must perform any conversion yourself.

Here is the source code for a PHP app I wrote a while back that converts the  JSON returned by the API to XML:

> http://quickmediasolutions.com/pastebin/1/stack2xml-source-code

It expects two `GET` variables to be set:

 - `method` - the API route to use (for example, `questions`)
 - `version` - the API version to use (for example, `2.0`)

These are in addition to any regular query string parameters expected by the API.