A previous investigation using steady state isotope switching in combination with DRIFTS demonstrated that surface formates exchange rapidly in a low temperature shift feed for the forward reaction. Transient decomposition of the pseudo-stable formate indicated that water autocatalyzes the forward shift reaction. In the present study, and including water in the RWGS feed (H-2 and CO2), surface formates were found to exchange more rapidly during RWGS over Pt/ceria than when a dry feed was used. An earlier claim suggested that Pt-CO and carbonate were intermediates, as they exchange rapidly during RWGS. However, in the present study of (CO2)-C-12 to (CO2)-C-13 switching, Pt-CO and carbonate exchange rapidly even in the absence of reaction. Furthermore, the formate exchange rate during RWGS in the absence of added water proceeds at a much slower rate, indicating that conclusions on the mechanism of forward shift cannot be inferred on the basis of dry RWGS results. (c) 2005 Elsevier B.V. All rights reserved.