The thermal decomposition of ammonium metavanadate(V) has been studied at atmospheric pressure, at hydrothermal, and high-pressure conditions, respectively. In a closed high-pressure system a redox reaction between vanadium(V) and ammonia takes place, to yield with increasing temperature the compounds (NH4)2V6O12(OH)2, VO2-δ(OH)δ, and V2nO2n-1 the composition of the product eventually approaching V2O3 at 1100 °C. All these compounds can easily be converted into V2O5 by heating in air. The mid-infrared spectra of the decomposition products obtained by slow heating of NH4VO3 in air, are virtually identical and show that ammonium hexavanadate(V), (NH4)2V6O16, is the most important, and perhaps only, intermediate in the “non-reduction” case. On further heating, (NH4)2V6O16 may not only loose ammonia and water to form V2O5 but could as well partially be reduced by ammonia. This could explain the diversity of intermediate “compounds” described in the literature. Hydrothermal reaction of NH4VO3 yielded (NH4)0.5V2O5, (NH4)2V3O8, and V2O3 at rather low temperatures.