Partial reduction of the ceria surface leads to the formation of bridging Type II OH groups, as reported previously. These were found to react with formic acid to yield bidentate formate and water, or with CO directly to form bidentate formate. In both WGS and formic acid decomposition via dehydrogenation whereby a high water/CO or water/formic acid ratio was utilized, a normal kinetic isotope effect was observed consistent with the involvement of formate C-H bond cleaving in the rate limiting step. In the current investigation, switching between a feed containing DCOOH to HCOOH led to an increase in the formic acid conversion, as well as a slight increase in the CO2 selectivity. The overall change in the CO2 yield was approximately 1.3, very close to the normal kinetic isotope effect of 1.4 reported in several studies of water gas shift on metal promoted ceria. By in situ DRIFTS, a slower D-formate decomposition rate versus H-formate in the presence of water was also observed during transient decomposition of the stabilized formate. Based on the identification of the same adsorbed reactive species and similar kinetic isotope effects, the results suggest an analogous mechanism operates for formic acid decomposition and water gas shift. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.