Thermal behavior of bulk CrO3 and supported CrO3/SiO2 catalysts on heating up to 1000degreesC in inert (Ar), oxidative (air, O-2) and reductive (H-2, CO) atmospheres was investigated by thermogravimetry (TG), differential thermal analysis (DTA) and mass spectroscopy (MS, SIMS). A two or three stage reduction pathway of unsupported CrO3 is or may be accompanied by simultaneous decomposition of intermediate CrOx phases (Cr3O8 and Cr5O12), which are eventually transformed into crystalline Cr2O3 at about 500degreesC. Entirely different behavior of supported CrO3/SiO2 is observed especially for low chromium content (up to 3% Cr). Oxidation at 500degreesC leads to catalyst surface covered with thermally stable chromate-like species CrO42-. These highly dispersed Cr-oxo species are anchored to silica substrate in the form of chromate esters. The increase in Cr content leads gradually to di-or polychromate species, amorphous and finally crystalline alpha-chromia phases. For low loaded CrOx/silica reversible interconversions Cr(VI) double left right arrow Cr(III) and Cr(VI) double left right arrow Cr(II) seem to be accepted when hydrogen or carbon monoxide are used as the reducing agents. In the case of CO-reduced sample the reactivity of Cr(II) leads to Cr(III) formation as a result of interaction with water or hydroxyl groups. Catalytically active Cr(II) and Cr(III) sites are involved in water gas shift reaction above 500degreesC. (C) 2003 Elsevier B.V. All rights reserved.