The interaction Of SO2 with ceria-zirconia model catalysts was studied using high-resolution synchrotron-based X-ray photoelectron spectroscopy (XPS). Epitaxial Ce1-xZrxO2(111) (x = 0.1 and 0.3) thin films (500-700 A in thickness) were grown by oxygen-plasma-assisted molecular beam epitaxy on single-crystal Y-stabilized ZrO2(111). Slightly defective surfaces were achieved by vacuum annealing at 900 K, and highly defective surfaces with 0 vacancies were obtained by 1.5-keV Ne+ sputtering. On the slightly defective Ce0.9Zr0.1O1.95(111) and Ce0.7Zr0.3O1.95(111) surfaces, the only products upon SO2 adsorption at 300 K are SO4/SO3 species, which gradually desorb from the surface between 300 and 900 K. SO2 adsorption on the heavily reduced surfaces results in different behavior. A complex set of compounds is observed during adsorption and thermal conversion processes. The Cedelta+ states (delta less than or equal to 3) play a dominant role in the adsorption Of SO2 and cleavage of S-O bonds. The relative amount of sulfur-derived adsorbates depends on the defect concentration: the higher the Cedelta+ concentration, the larger the amount of formed atomic S. On Ce0.9Zr0.1O1.50(111) and Ce0.7Zr0.3O1.50(111) surfaces, sulfate, sulfite, and atomic sulfur species coexist at 300 K. The Zr cations increase the stability of the SO4/SO3 groups on the oxide surface. Thermal annealing (for Ce0.9Zr0.1O1.50(111), > 400 K; for Ce0.7Zr0.3O1.50(111), > 700 K) leads to the formation of oxy-sulfides (M2O2S, M= Ce.or Zr), which are converted from either sulfate or sulfite. The formation of the oxy-sulfides produces a substantial shift in the Zr 3d core levels.