The mechanism of the catalytic reduction of SO 2 by CO and COS over lanthanum oxysulfide was studied using a combination of temperature-programmed reaction coupled with mass spectrometry (TPR/MS) and feed-perturbation step-analysis techniques. The results showed that COS interacts more readily with lanthanum oxysulfide than CO does. The interaction of COS with the lanthanum catalyst was different from that of CO. In addition, COS preferentially reacted with SO 2 to form CO 2 and sulfur instead of following the disproportionation (to form CO 2 and COS) and decomposition (to form CO and sulfur) reactions. The redox mechanism is not the main reaction route for the reduction of SO 2 by CO and COS over lanthanum oxysulfide. SO 2 was strongly adsorbed by the oxysulfide and was retained in the oxysulfide even when heated in an inert gas stream, while COS was very reactive and did not remain long in the oxysulfide. These results suggest that COS has access to certain specific active sites in the oxysulfide and the catalytic reduction of SO 2 by COS possibly proceeds via adsorbed COS reacting with pools of SO 2 adspecies to form CO 2 and sulfur.