The catalytic deactivation and the simultaneous formation of compounds in industrial sulfuric acid catalysts and their molten salt-gas model systems M(2)S(2)O(7)/V2O5-SO2/O-2/SO3/N-2 (M = Na, K, Cs) have been studied by combined activity measurements and in situ ESR spectroscopy at temperatures up to 500 degrees C. The applied gas compositions were unconverted, and 50 and 90% preconverted standard feed gas containing 10% SO2, 11% O-2, and 79% N-2. This covers the conditions of all beds in an industrial SO2 converter, without interstage absorption of SO3. The temperature of deactivation was shown to decrease with increased degree of preconversion of the feed gas, increased mixing of the alkali promoters, and decreased vanadium content in the catalysts and model systems. The precipitation of V(II), V(IV), and mixed valence V(IV)-V(V) compounds was observed below the onset temperature of the catalyst deactivation. The salts have been isolated under operating conditions from model melts and identified by microscopy and spectroscopic methods. The V(IV) compounds were characterized by ESR spectroscopy and their temperature of decomposition was found to be in the range 450-500 degrees C. Based on the characteristic ESR spectra, the V(IV) compounds causing the deactivation of working industrial catalysts could be identified in situ. Finally it was found that the restoration of deactivated catalysts requires heating to around 500 degrees C, where the low-valence vanadium compounds decompose and reoxidize to V(V).