The dissociation of CO and H2O during the water-gas shift reaction (WGSR) on the 973 K-carburized 4.8 and 8.5 wt% Mo/Al2O3 catalysts at 423 K was studied by in situ mass and infrared spectroscopies. It was found that both CO and H2O were dissociated on the basis of the formation of H-2, (CO)-C-13-O-18, (CO2)-C-13-O-18 and (COO)-C-13-O-18-O-16 after the injection of (H2O)-O-18 into the (CO)-C-13-O-16 stream. Regarding the C-12- and O-16-labeled atoms, (CO2)-C-12-O-16. (COO)-C-12-O-16-O-18 and (CO2)-C-12-O-18 were not formed, but (H2O)-O-16, (CO)-C-12-O-16, (CO)-C-12-O-18 and (CO2)-C-13-O-16 were formed. The (H2O)-O-16, (CO)-C-12-O-16 and (CO2)-C-13-O-16 were formed by the reaction of the dissociated O-16 of (CO)-C-13-O-16 with the surface carbon or the lattice carbon atom of the Mo oxycarbide. The infrared spectroscopy results for the injection of H2O into the (CO)-O-18 flow showed that (CO)-O-18 had no effect on forming the formate group. The formate and carbonate which were probably formed during the CO2 hydrogenation were observed when using the 973 K-carburized Mo/Al2O3 catalyst and alumina in a stream of CO. The WGSR on the Mo oxycarbide of the carburized Mo/Al2O3 catalyst followed the redox route together with the dissociation-association mechanism. (C) 2011 Elsevier B.V. All rights reserved.