In this study, the synergistic effect observed in the SO2 reduction by CO over SnO2-ZrO2 catalysts was investigated in detail. From the reactivity tests of the various catalysts, it was established that the applicable mechanism of this study was different from the conventional mechanisms such as the redox and COS intermediate mechanisms. It could be explained by postulating that the synergistic effect of the Sn-Zr-based catalyst was obtained in the SO2 reduction by CO as the following mechanistic pathway: In the first step involving the redox mechanism, SO2 was converted into elemental sulfur and other by-products such as COS because of the lattice oxygen mobility afforded by the redox properties of the SnO2 catalyst. In the second step involving the COS intermediate mechanism, COS produced in the first step was the more effective reducing agent than CO and reacted with SO2 over the ZTO(2) catalyst, and then the productivity of elemental sulfur was improved. At that time, the reaction rate of SO2 + 2COS -> 3S + 2CO(2) must have been faster than that of CO + S - COS. The high reactivity of the SnO2-ZrO2 (Sn/Zr = 2/1) catalyst at low temperature was thus caused by a synergistic effect resulting of the simultaneous interaction between the redox mechanism and COS intermediate mechanism. (c) 2007 Elsevier B.V. All rights reserved.