The effect of H2O on the catalytic reduction of SO2 and NO on La2O2S was Studied using temperature-programmed reaction coupled with fast mass spectrometry, powder X-ray diffraction and X-ray photoelectron spectroscopy. It is found that La2O2S can be completely and irreversibly deactivated in the presence of H2O at 700 degrees C when NO/SO2 is sufficiently high (similar to 1.0). This is caused by the formation of a layer of inactive and stable La2O2SO4 on the oxysulfide. When NO is absent or NO/SO2 is low (similar to 0.4), H2O inhibits the reduction and shifts the selectivity from sulfur to H2S. While the causes of the deactivation can be attributed to the Reverse Claus Reaction between H20 and sulfur in the oxysulfide, the competitive hydrolysis of the COS intermediate and the competitive adsorption of H2O, the shift in selectivity to H2S is attributable to the former two factors. (c) 2007 Elsevier B.V. All rights reserved.