Both the X-ray photoelectron spectroscopy (XPS) smd temperature-programmed reaction, coupled with mass spectrometry (TPR/MS), are used to study the formation of carbonyl sulfide in the reduction of sulfur dioxide on lanthanum oxysulfide catalyst. It was found that the lattice sulfur of the oxysulfide is released and reacts with carbon monoxide to form carbonyl sulfide when the oxysulfide is heated. The oxysulfide is postulated to form sulfur vacancies at a temperature lower than that for the formation of carbonyl sulfide and atomic sulfur is released in the process. The atomic sulfur can either enter the gas phase and leave the oxysulfide catalyst or react with carbon monoxide to form carbonyl sulfide.