The dehydrogenation of methanol on Pt(1 1 1)/water and Pt(1 1 1)/vacuum interfaces was investigated by using ab initio theoretical calculations. Possible reactive intermediate structures were optimized, and energetically favorable reaction pathways were traced at these two interfaces. When all products formed through the dehydrogenation remained on a Pt(1 1 1) surface, the reaction energy of a whole dehydrogenation was smaller at the Pt(1 1 1)/water interface than that at the Pt(1 1 1)/vacuum interface by about 0.7 eV. (C) 2003 Elsevier B.V. All rights reserved.