The reduction reaction of SO₂ by CO over La₂O₃-impregnated on TiO₂ catalysts was studied. CO conversion was increased and COS selectivity was decreased with increased impregnation of La₂O₃. The adsorption of CO on the catalyst was too small to be measured at temperatures from 25 ℃ to 400 ℃. The adsorption of SO₂ was increased with increasing La₂O₃ impregnation, and that of O₂ also showed the same tendency. When the catalyst was exposed to oxygen during the reaction, the catalytic activity was decreased significantly, but the reduced catalytic activity was restored under the reactant. From these experimental results oxygen lattice vacancy was suggested as the active site of this reaction and the improvement of catalytiz activity due to La₂O₃- impregnation was explained with the high probability of oxygen vacancy formation to La₂O₃.