In previous work, steady-state isotope switching was conducted on Pt/ceria with H2O and D2O, and a normal kinetic isotope effect was observed. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicated that the surface coverage of H-formate was more limited by the WGS rate than that of D-formate, linking the rate limiting step to the breaking of the formate C-H bond. In this work, the transient response of the decomposition of H and D formates was followed thermally and in the presence of H2O and D2O. A normal kinetic isotope effect was again obtained, in agreement with our previous findings, and those reported by Shido and Iwasawa. Consistent with their work, water was found to assist in promoting the forward decomposition of surface formates to hydrogen and unidentate carbonate, the precursor to CO2. (C) 2004 Elsevier B.V. All rights reserved.