To investigate the possibility of simultaneous catalytic reduction of NO and SO2 by CO, reactions of NO, NO-CO, and NO-SO2-CO were performed on gamma -alumina-supported sulfides of transition metals including Co, Mo, CoMo and FeMo. NO was decomposed into N2O and N-2 accompanied with the formation of SO2; this Serious oxidation of lattice sulfur resulted in the deactivation of the catalysts. The addition of CO to the NO stream suppressed SO2 formation and yielded COS instead. A stoichiometric conversion of NO and CO to N-2 and CO2 was observed above 350 degreesC on the CoMo and the FeMo catalysts. Although the CO addition lengthened catalyst life, it was not enough to maintain activity. After the NO-CO reaction, an XPS analysis showed the growth of Mo6+ and SO42- peaks, especially for the sulfided FeMo/Al2O3; the FeMo catalyst underwent strong oxidation in the NO-CO reaction. The NO and the NO-CO reactions proceeded non-catalytically, consuming catalyst lattice sulfur to yield SO2 or COS. The addition of SO2 in the NO-CO system enabled in situ regeneration of the catalysts; the catalysts oxidized through abstraction of lattice sulfur experienced anew reduction and sulfurization through the SO2-CO reaction at higher temperature. NO and SO2 were completely and catalytically converted at 400 degreesC on the sulfided CoMo/Al2O3. By contrast, the sulfided FeMo/Al2O3 was easily oxidized by NO and hardly re-sulfided under the test conditions. Oxidation states of the metals before and after the reactions were determined. Silica and titania-supported CoMo catalysts were also evaluated to study support effects.