Transition and rare earth metal vanadates are potential active phases for the selective catalytic reduction (SCR) of nitric oxide by ammonia for exhaust gas emission control. In this work, various metal vanadates mixed with SiO2-WO3-TiO2 (TWS) were compared to vanadia-based SCR catalysts. FeVO4-based catalysts were found to be the most active metal vanadates, followed by CeVO4 and ErVO4. In depth analysis using XRD, BET, H-2-TPR, DRUV and DRIFTS demonstrated that the vanadates partly decomposed above 600-750 degrees C to the corresponding single metal oxides, the decomposition temperature correlating with their relative stability. The activity and the estimated fraction of freed VOx from the vanadate decomposition strongly correlated with vanadia-based catalysts at comparable V-loading. Based on these findings, the enhanced thermal stability of the vanadate-based catalysts was correlated to an overall lower amount of free VOx species compared to vanadia-based catalysts. The released VOx species are responsible for the activity of the metal vanadate-based SCR catalysts and are of similar nature to those of vanadia-based catalysts. Therefore, the claimed high temperature stability advantage of supported metal vanadates is merely an effect of the degree of vanadate decomposition and is not related to their intrinsic stability.