Mixed oxides of Co3O4-TiO2 have shown the highest catalytic activity for the reduction of SO2 by CO among catalysts that have been developed so far. Almost zero conversion was observed with cobalt alone, whereas a high conversion was obtained with TiO2 especially at high temperatures. There existed a strong synergistic promotional effect in the conversion of SO2 when cobalt was mixed with TiO2. The synergistic effect observed with mixed oxides is caused by simultaneous contributions from two different reaction routes via COS intermediate mechanism and modified redox mechanism. The synergistic effect that is caused by the COS mechanism has a smaller amount of contribution in the conversion increase and remains almost constant with an increase in the reaction temperature. A larger portion of the synergistic effect is contributed from the modified redox mechanism especially at low temperatures, but the effect disappears at temperatures above 450 degrees C, It is found that the introduction of cobalt into TiO2 produces COS by the reaction between sulfided CoS2 and CO even at low temperatures. The COS intermediate can react with SO2 to produce an additional sulfur via the COS intermediate mechanism, and also behaves as a strong reductant to keep oxygen vacancies on the TiO2 in a high concentration for the production of sulfur via modified redox mechanism.