The hydrogenation of CO on Mo and CoMo carbide catalysts was evaluated by quadrupole mass (MS) and in situ infrared (FTIR) spectroscopic measurements. The difference in the catalytic properties of the Mo and CoMo carbides and the role of the Co atom in the catalytic activity were determined. The surface-adsorbed-species-derived CO and H-2 were estimated using (CO)-O-18 for the 973 K-carburized CoMo catalyst by MS spectrometry in flowing 2%CO/61%H-2 with a He balance at 453 K, showing that the (CO)-O-18 was dissociated to form (H2O)-O-18 and (COO)-O-18-O-16. The H-2 introduction formed a significant amount of CH4 together with a small amount of (H2O)-O-16 without (H2O)-O-18. As a result, CO was dissociatively adsorbed on the CoMo carbide by the H-2 introduction to form the OH group and CH4. A slight amount of CO2 was formed during the reaction on the CoMo carbide catalyst, which was different from the formation of CO2 on the 973 K-carburized Mo catalyst. The introduction of Co to Mo2C prevented the water-gas shift reaction. The CH4 gas band at 3016 cm(-1) was detected and significantly increased after the appearance of the bands for the surface OH species at 3570-3760 cm(-1) on the surface of the 873 K-carburized Mo catalyst. The formation of H2O preceded the formation of CH4. (C) 2013 Elsevier B.V. All rights reserved.