The adsorption and decomposition of H2S on CeO2 (111) surfaces were studied using the first-principles calculations based on the density functional theory and the pseudopotential method. The adsorbed S species on CeO2 surfaces were classified as molecular H2S, SH, and atomic S. The molecular H2S was mostly associated with surface Ce sites. The adsorption of HS was more stable on O sites, and atomic S specie was bridged with Ce and O sites. The first dehydrogenation step of H2S on CeO2 (111) surface had a small barrier (0.16 ev), and the second step was 0.28 ev. The differences adsorption and decomposition of H2S on CeO2 (111) surface and defective CeO2 (111) surface were compared. This is an abstract of a paper presented at the 231st ACS National Meeting (Atlanta, GA 3/26-30/2006).