Water-gas-shift (WGS) reaction rates have been measured on Pd/ceria, Ni/ceria, Fe/ceria, Co/ceria, ceria, and Pd/silica. Pd/ceria exhibited much higher activities than either ceria alone or Pd/silica, demonstrating a cooperative effect between Pd and ceria. Pd/ceria and Ni/ceria showed essentially the same activities and were much more active than either Co/ceria or Fe/ceria. Reaction orders on Pd/ceria were approximately zeroth-order in CO, half-order in H2O, inverse-half-order in CO2 and inverse-first-order in H-2. Diffuse reflectance, FTIR measurements on Pd/ceria indicate the ceria exists in a reduced state under WGS conditions and is covered by carbonate species that are removed only by reoxidation of ceria. The implications of these results for improved WGS catalysis are discussed.