The catalyst deactivation and the simultaneous formation of compounds in commercial SO2 oxidation catalysts have been studied by combined activity measurements and in situ EPR spectroscopy in the temperature range 350-480 degrees C. The applied feed gas compositions were 0.2% SO2, 4.5% O-2, 15% CO2, and similar to 80% N-2; and 0.2% SO2, 4% O-2, 7% H2O, 14% CO2, and similar to 75% N-2. These conditions simulate hue gas from coal fired power plants with and without water. It was shown that in both dry and wet hue gas, the V(IV) compound K-4(VO)(3)(SO4)(5) precipitates below the temperature of catalyst deactivation. However, for a given catalyst composition, the deactivation temperature in the wet hue gas was strongly influenced by the structure of the support, whereas similar behaviour was not observed in the dry hue gas. Thus, one of the studied catalysts, with a particular pore structure, most probably a V(III) compound, precipitated first during deactivation in the wet flue gas, while the V(IV) compound, K-4(VO)(3)(SO4)(5), only precipitated with a further lowering of the temperature. Water vapour seems therefore to have a significant influence on the performance of the SO2-oxidation catalyst. A qualitative explanation is given for the formation of both V(IV) and V(III) compounds during deactivation.