When ammonium metavanadate is roasted at 225 °C, ammonia and water are eliminated in the ratio 2:1 and an intermediate compound identified by weight loss and chemical analysis as (NH4)2•O•V2O5• is formed. An X-ray diffraction pattern for this material is reported. Transpiration experiments have been used to establish the free energy of this reaction. The second stage of the decomposition involves an endothermic and an exothermic process, both of which occur with the production of a gaseous product. The liberation of ammonia is not quantitative, and its oxidation on the vanadium pentoxide, in the last stage of decomposition, is suggested as a possible source of the exothermic heat.