The interaction of carbon monoxide with Fe2O3-Ce0.5Zr0.5O2 was investigated by the adsorption investigation under isothermal CO/H-2 exposure and temperature-programmed desorption (TPD), as well as by in-situ diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS), in order to understand the carbon monoxide formation during the cyclic water gas shift reaction. When the Fe2O3-Ce0.5Zr0.5O2 catalyst was exposed to a mixture of CO and H-2, the majority of the surface species depicted by the DRIFT analysis were associated with carbonates. The carbonates were essentially stable in a He atmosphere at temperatures ranging from 60 to 450 degrees C. An increase in the temperature leads to the conversion of the carbonates, where the oxygen comes from the catalyst lattice. Investigations carried out using a H2O/He mixture showed that carbon oxides were produced during the interconversion of carbonate species on the catalyst surface with steam. The main gas component produced during the TPD of an Fe2O3-Ce0.5Zr0.5O2 catalyst in the temperature interval of 60-500 degrees C was carbon dioxide. The amount of carbon oxides produced decreased at elevated temperatures. The steady-state isotopic (CO)-O-18 experiments revealed that the Boudouard reaction occurred at temperatures higher than 350 degrees C. The carbon deposits which were formed on the catalyst surface during the reduction step through the Boudouard reaction led to CO formation during the successive re-oxidation step. (c) 2007 Elsevier B.V. All rights reserved.