Sulfidization of lanthanum oxide and hydroxide to the corresponding oxysulfide was studied using simultaneous thermogravimetry/differential scanning calorimetry coupled with mass spectrometry (TG/DSC/MS) and temperature-programmed sulfidization coupled with mass spectrometry (TPS/MS) techniques. It was found that the dehydroxylation of lanthanum hydroxide in the presence of carbon monoxide was a critical step in the sulfidization process if the final product is to be the oxysulfide. The results of this study provide further support to the sulfidization mechanism proposed by Ma et al.