The thermal decomposition course of ammonium paratungstate (APT) in hydrogen was studied using thermogravimetric and differential thermal analyses. X-ray diffractometry, infrared spectroscopy and diffuse reflectance spectroscopy were used to characterize and identify the intermediate solid products. It was found that ammonium paratungstate decomposes to tungsten metal W-0 through five main steps encompassing different tungsten intermediate compounds. Ammonium tungsten bronze (NH4)(0.33)WO3, which precedes the formation of WO3, is relatively the most stable intermediate (250-550 degrees C) encountered through reduction of APT to tungsten metal W-0. The influence of hydrogen spillover on the reduction behavior commences to be effective just after formation of the bronze intermediate.