V2O5-K2SO4-SiO2 catalysts were studied by TG, DTA, TPR and XRD methods. During activation NH4VO3 and (COOH)(2), used for impregnation of the support, decompose in air below a temperature of 350 degrees C. Over 350 degrees C oxidation of vanadium up to V5+ and changes in the catalyst reducibility occur. In TPR using pure hydrogen, the freshly activated catalyst is reduced in three steps. The first peak with a maximum around 430 degrees C is a result of reduction of the vanadium oxide species. The second and the third TPR peaks correspond mainly to the reduction of sulfate species to H2S. It was found that vanadium oxides noticeably enhanced the reduction of sulfate to H2S. Similar TPR profiles were found after catalytic tests (vapor phase oxidation of toluene by air) and after reduction of the same catalyst by H-2, and its reoxidation by air in some particular conditions. By increasing temperature and prolonging time of activation, or by prolonging time-on-stream of toluene oxidation the reducibility of vanadium oxides decreases and the easily reducible part of sulfate species is converted into a less reducible part.