The compound, (NH4)[VO(O2)2(NH3)], thermally decomposes to ammonium metavanadate, which then decomposes to vanadium pentoxide. Using a heating rate of 5 deg·min−1, the first decomposition step occurs between 74° and 102°C. The transformation degree dependence of the activation energy (α-E) is shown to follow a decreasing convex form, indicating that the first decomposition step is a complex reaction with a change in the limiting stage of the reaction. Infrared spectra indicated that the decomposition proceeds via the gradual reduction of the ratio of the ‘(NH4)2O’ to ‘V2O5’ units from the original 1∶1 ratio in ammonium metavanadate, which may be written as (NH4)2O·V2O5, to V2O5.