A series of MO_x(transition metal oxides)/TiO₂ powder mixture were studied as catalysts for the reduction of SO₂ by CO to elemental sulfur. Among these catalysts, SnO₂/TiO₂ was found to be the most effective catalyst for the reaction. When the catalyst was a mixture of SnO₂ and TiO₂ the activity increased drastically. Experimental evidence of the formation of a stable COS was obtained from a two-stage reactor system and TPR. XRD data of the used catalyst showed the presence of SnS₂, SnS, and TiO_2-x phases. In the reaction system, an equilibrium is established between SnS₂ and SnS, which acts as a medium for sulfur supply in the formation of COS. From the above results, a mechanism based on formation of COS intermediate and catalyst bifunctionality is proposed for the SO₂ reduction by CO.