Operando diffuse reflectance FTIR spectroscopy (DRIFTS) combined with isotopic exchange techniques were used to investigate the bands located in the carbonyl-stretching region observed over a Au/CeZrO4 catalyst under water-gas shift (WGS) conditions. The use of (CO)-C-12 and (CO)-C-13 in the feed was essential to distinguish between Au-carbonyl bands and an electronic transition band associated with Ce-III centers and to allow correct deconvolution of the carbonyl signal. The very active WGS catalyst used here as an example displays three main types of carbonyl species under reaction conditions, tentatively assigned to CO adsorbed on positively charged, neutral, and negatively charged An entities. The DRIFTS data collected in the present work characterize the catalyst under reaction conditions and thus circumvent the difficulties associated with possible catalyst modifications during and after use. The possible presence of the electronic absorption (the maximum of which is at ca. 2120-2140 cm(-1)) due to the presence of Ce-III centers always should be considered when analyzing metal-carbonyl IR bands. It also is crucial to realize that this band also may form in situ by reduction of the Ce-IV while under reaction conditions or when the sample is titrated with CO. (C) 2008 Elsevier Inc. All rights reserved.