Selective oxidation of methanol on oxygen-modified Mo(112) was investigated by temperature-programmed reaction (TPR) and under the catalytic reaction conditions at constant pressures of CH3OH and O-2 (10(-6)-10(-5) Pa). Low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) were also used. A Mo(112)-(1 x 2)-O surface (theta(0) = 1.0) has a characteristic structure with alternating one-dimensional Mo rows of Mo-2C and MONC The Mo-2C rows are affected by preadsorbed oxygen atoms ((1 x 2)-O), whereas the MONC rows served as the adsorption and reaction sites. Formaldehyde (H2CO) was a major product with 50% selectivity in TPR of methanol on the Mo(112)-(1 x 2)-O surface, whereas CH4, H-2, C(a), and O(a) were the products at lower oxygen coverages than the (1 x 2)-O coverage. Extra oxygen species on the MONC row of Mo(112)-(1 x 2)-O increased the selectivity to formaldehyde to 88% and decreased the activation energy for the rate-limiting C-H bond scission of methoxy species. In a constant flow of methanol alone, the reaction proceeded for several cycles, but eventually the surface was deactivated by accumulation of carbon. Selective catalytic oxidation of methanol successfully proceeded on Mo(112)-(1 x 2)-O in a constant flow of O-2 and CH3OH without deactivation because of the presence of the extra oxygen atoms on the Mo-NC rows during the reaction.