Density functional theory (DFT) was employed to investigate the behavior of Mo carbides in the water-gas-shift reaction (WGS, CO + H(2)O --> H(2) +CO(2)). The kinetics of the WGS reaction was studied on the surfaces of Mo-terminated Mo(2)C(001) (Mo-Mo(2)C), C-terminated Mo(2)C(001) (C-Mo(2)C), and Cu(111) as a known active catalyst. Our results show that the WGS activity decreases in a sequence: Cu > C-Mo(2)C > Mo-Mo(2)C. The slow kinetics on C-Mo(2)C and Mo-Mo(2)C is due to the fact that the C or Mo sites bond oxygen too strongly to allow the facile removal of this species. In fact, due to the strong O-Mo and O-C interactions, the carbide surfaces are likely to be covered by O produced from the H(2)O dissociation. It is shown that the O-covered Mo-terminated Mo(2)C(001) (O_Mo-Mo(2)C) surface displays the lowest WGS activity of all. With the Mo oxide in the surface, O_Mo-Mo(2)C is too inert to adsorb CO or to dissociate H(2)O. In contrast, the same amount of O on the C-Mo(2)C surface (O_C-Mo(2)C) does not lead to deactivation, but enhances the rate of the WGS reaction and makes this system even more active than Cu. The good behavior of O_C-Mo(2)C is attributed to the formation of a Mo oxycarbide in the surface. The C atoms destabilize O-poisoning by forming CO species, which shift away from the Mo hollow sites when the surface reacts with other adsorbates. In this way, the Mo sites are able to provide a moderate bond to the reaction intermediates. In addition, both C and O atoms are not spectators and directly participate in the WGS reaction.