Lanthanum oxide by itself has been reported to be inactive for the reduction of SO2 by CO; however, we discovered that La2O3 can be activated in a reaction gas atmosphere if previously hydrated. The active phase of the activated lanthanum oxide was identified to be lanthanum oxysulfide by using X-ray diffraction. This showed that the activation was in essence a sulfidization of the oxide. An activation mechanism is proposed based on the surface acidic and basic properties resulting from the dehydroxylation of the lanthanum hydroxide.