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. (C) 2003 Elsevier B.V. All rights reserved.