TiO2-promoted cobalt sulfides, which were prepared by pre-sulfiding with H2S/H-2 the titanium-cobalt oxides prepared by coprecipitation method in order to get an intimate contact between them, were studied for the separate and simultaneous reduction of SO2 and/or NO by CO. Physicochemical characterizations of the catalysts before and after reactions were conducted to disclose the reaction mechanism by means of XRD, BET, XPS and transient MS techniques. It is found that TiO2-promoted cobalt sulfides were more active catalysts for the reduction of SO2 by CO than the mechanical mixtures of the components. There exists a strong synergistic promoted effect, which can be explained based on a COS enhanced redox mechanism proposed in the work. Furthermore, not only the addition of NO had little effect on this reaction, but also a combined reduction of SO2 and NO by CO occurred in a gas mixture comprising very low concentrations of them. The reduction of NO is explained by a NO decomposition mechanism over sulfides with the formation of by-product, SO2, based on the MS results. There also exists a synergistic effect between cobalt sulfide and TiO2. The produced SO2 can further be reduced over TiO2 to sulfur, which might spill over to cobalt sites to regenerate cobalt sulfide. The SO2 + NO + CO reaction was proposed to proceed via two separate reactions of SO2 + CO and NO decomposition. However, the two reactions are interrelated. The reaction between CO and cobalt sulfide to produce COS intermediate is a necessity for the further reduction reaction of SO2 on TiO2; while SO2, one of the products of NO decomposition on cobalt sulfide, is recovered to sulfur on TiO2 by SO2 + CO reaction. Lastly, CO2 was found to promote the production of COS, but has little effect at levels around 1000 ppm. (C) 2002 Elsevier Science B.V. All rights reserved.