The water-gas-shift (WGS, CO + H2O -> H-2 + CO2) reaction was studied on the Ni2P(001) surface using a combination of experimental and theoretical methods. Our experimental measurements show that Ni2P(001) displays an activity larger than that of Ni(100) or even Cu(100), which is the best metal catalyst for the WGS process. The good behavior of Ni2P is associated with the Ni oxy-phosphides formed as a result of strong O <-> P interactions. Under reaction conditions, most of the P sites of Ni2P(001) are covered with oxygen. The addition of Cs to the nickel phosphide surface increases the saturation coverage of oxygen and enhances catalytic activity. As in the case of a [NiFe] hydrogenase enzyme, the active sites of O/Ni2P(001) involve the combination of a metal and a light atom. This configuration of the active sites leads to significant changes in the reaction mechanism with respect to that on Ni(100) or Cu(100). The O atoms on the Ni2P(001) surface facilitate the WGS reaction in both direct and indirect ways. On one hand, O helps to lower the barrier for water dissociation; on the other hand, it also deactivates the Ni sites in the surface to provide moderate bonding to the adsorbates, and the barriers for each elementary step in the WGS process become lower than 1.2 eV. Our results imply that the high performances of catalysts in the WGS rely heavily on the cooperation between oxygen and metal centers with moderate activity. Published by Elsevier Inc.