The partial oxidation of methanol to formaldehyde on clean and oxygen-precovered Cu(100) and Cu(110) has been studied by density functional theory calculations within the generalized gradient approximation. We have determined the geometric and electronic structure of the reaction intermediates. Methanol and formaldehyde are only relatively weakly bound to copper, whereas methoxy is strongly chemisorbed. Still, we have identified a highly deformed formaldehyde species strongly interacting with Cu(100) and Cu(110). The reaction paths have been determined by the nudged elastic band method. It turns out that the rate-limiting step is the dehydrogenation of methoxy to formaldehyde, which is hindered by a significant activation barrier. Although dosing with oxygen does not reduce this barrier, it still facilitates this reaction by stabilizing the methoxy intermediate on the Cu surface and causing the removal of surface hydrogen via water desorption. The strain of the copper substrate leads to enhanced total binding energies of all reaction intermediates, but there is no clear trend, as far as the height of reaction barriers as a function of the strain is concerned. (c) 2005 Elsevier Inc. All rights reserved.