A trace amount of oxygen in H-2 promotes a new type of direct hydrogenation reaction of adsorbed CO on Ni(100) surface. The formation of H(x)CO(y) was suggested by high resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). HREEL spectra showed the formation of surface hydroxyl (OH) and the C-H bonds of H(x)CO(y) species but no carbonyl (C=O) loss peak was detected although thermal desorption yielded large amount of CO. The H(x)CO(y) undergoes the decomposition at 400-450 K on the hex-OH Ni(100) surface, which yielded CO, CO2, H-2 and H2CO. It was confirmed that no C-H bond formation occurs on c(2 x 2)-O, p(2 x 2)-O Ni(100) and hex-OH Ni(100) as well as on clean Ni(100) surfaces. This fact indicates that the gas phase oxygen may induce the direct hydrogenation of CO to form H(x)CO(y), which is analogous to the hydrogenation of 0 to form hex-OH on Ni(100).