In this study, the thermal decomposition mechanism of ammonium phosphomolybdate hydrate in air and inert gas atmospheres was determined and the intermediate and final products formed during the thermal decomposition were characterized using TGA, DTA, FTIR, XRD, EDS and EGA methods. In the first decomposition step, in both atmospheres, ammonium phosphomolybdate hydrate lost only its crystal water. At the following decomposition steps, ammonium phosphomolybdate hydrate acted differently in air and inert gas atmospheres. In air, in the second decomposition step, stoichiometric phosphomolybdenum oxide was formed as the final solid product. The ammonia gas evolved was oxidized with the oxygen present in air to form nitrogen and water. In inert gas atmosphere, in the second decomposition step, a portion of the ammonia gas evolved reduced the stoichiometric oxide formed (phosphorus hydrogen molybdenum oxide) to produce the nonstoichiometric oxide (phosphorus hydrogen molybdenum suboxide). The rest of the ammonia gas was released from the system partly undissociated and partly dissociated as nitrogen and hydrogen. In inert gas atmosphere, in the third decomposition step, the phosphorus hydrogen molybdenum suboxide formed lost its water to produce the phosphomolybdenum suboxide. It was also observed that phosphomolybdenum suboxide had good serniconductive property. The activation energies for the first and second decomposition steps of ammonium phosphomolybdate hydrate were determined in nonisothermal conditions. (c) 2006 Elsevier B.V. All rights reserved.