In this investigation. isotope switching from H2O to D2O was used to validate the possibility that the decomposition of formate could be the rate limiting step for the water gas shift mechanism over metal promoted ceria, as was proposed earlier by Shido and Iwasawa. The surface species were followed stepwise by in-situ DRIFTS, while the conversion was monitored by GC to quantify the conversions. A normal kinetic isotope effect was observed, consistent with a link between the activation energy barrier of the rate limiting step and the breaking of the formate C-H bond. (C) 2004 Elsevier B.V. All rights reserved.