Oxygen adsorption on Mo2C(0 0 0 1) has been investigated with angle-resolved photoemission spectroscopy (ARPES). When the surface is reacted with O-2, the O 2p-induced states are formed at 4.1 and 5.3 eV at the (Gamma) over bar point. The emissions around the Fermi level are also intensified by oxygen adsorption, which is due to the formation of a partially filled state. It is found that the reactivity of the surface toward H2O adsorption is much enhanced by pre-adsorption of oxygen. The reactivity is found to be maximized at theta(O) similar to 0.2. (c) 2008 Elsevier B.V. All rights reserved.