The interaction of sulfur with ceria under highly reducing conditions was investigated. The phase boundary between CeO1.83 and Ce2O2S was determined for temperatures between 873 and 1073 K. This data was used to derive an empirical equation for DeltaG(f)degrees of Ce2O2S in this temperature range. This equation along with thermodynamic data for cerium oxides and sulfides obtained form the literature was used to predict Ce-O-S phase diagrams at 873 and 973 K. These phase diagrams provide insight into the mechanism of the deactivation of ceria-based catalysts by sulfur under reducing conditions. (C) 2002 Elsevier Science B.V. All rights reserved.