Sn0.5Ti0.5O2 shows excellent catalytic performance both for the CO-SO2 reaction and the CO-SO2NO reaction. At 350 degreesC, 525 ppm SO2/520 ppm NO/2085 ppm CO, SV = 3000 h(-1), the conversion of SO2 is nearly complete in the CO-SO2 reaction and above 89% in the CO-SO2-NO reaction; NO conversion is above 98% in the latter reaction. The selectivities of S and N-2 are both close to 100%. SO2 shows a significant promoting effect on the activity of the Sn0.5Ti0.5O2 catalyst for NO reduction by CO. Combining transient response experiments, catalytic tests and TPD results, we propose a SO2-assisted NO CO reaction concept. The existence of a surface sulfur species, which was formed during the CO-SO2 or CO-SO2-NO reaction, is proved by XPS analysis. It is the active site for NO reduction in the CO-SO2-NO reaction, and through which SO2 accomplishes its promoter role. On the basis of the results obtained, the SO2-assisted redox mechanism of simultaneous reduction of SO2 and NO by CO is proposed.