Deactivation processes of commercial and prototype sulphuric acid catalysts have been investigated in-situ both in highly converted and unconverted feed gases, using electron paramagnetic resonance (EPR) spectroscopy. The investigation revealed that the deactivation of catalysts for SO2 oxidation is strongly dependent on physical and chemical parameters such as the pore structure of the support, the type of the alkali promoter as well as the SO2 and SO3 partial pressures. A hysteresis effect in catalytic activity could be observed during reheating of the deactivated catalysts. in this context indirect evidence was found of deactivation by the formation of vanadium(III) compounds. The various vanadium compounds in the oxidation state (IV) and with mixed valence (IV-V), formed during deactivation, have been characterized by EPR. The investigations showed no change in the vanadium oxidation state in the catalytic cycle under steady-state conditions, and at high SO2 partial pressure even at low temperatures, indicating that the so-called ’associative mechanism’ probably dominates in the catalytic SO2 oxidation. A statistical lattice model was applied to describe the crystallization of binuclear complexes in the melt of sulphuric acid catalysts during deactivation while taking into account parameters such as temperature, the composition of the catalyst and SO2 conversion.