The effect of catalyst loading, composition, and calcination temperature on NH3 selective catalytic reduction (SCR) activity was investigated for metal vanadates of the type FexAl1-xVO4 (0 <= x <= 1, MeVO4) supported on TiO2-WO3-SiO2 (TWS). The optimized catalyst (4.5 wt % FeVO4/TWS calcined at 650 degrees C) showed an enhanced NOx reduction activity compared to the reference material 2.3 wt % V2O5/TWS. An activation effect was observed above a calcination at 600 degrees C and was investigated by means of X-ray diffraction, Brunauer-Emmett-Teller, diffuse reflectance infrared Fourier transform, and X-ray absorption near edge structure analyses. It was shown that the activation is due to the decomposition of FeVO4. VOx species disperse and migrate to the support material, while the iron species sinter to Fe2O3 particles. We provide strong evidence that the active species responsible for NH3-SCR in a FeVO4/TWS catalyst is not FeVO4 but VOx species that possess a similar coordination environment to the VOx species on conventional V-based catalysts. Due to the remarkable effect of the activation upon calcination, a very active and thermally stable SCR catalyst up to 700 degrees C was obtained.