A quantum chemical investigation of methane activation on reduced and partially oxidized Ni(100) surfaces has been performed. For all surfaces, methane activation has been assumed to proceed by dissociative adsorption at an on-top site. The activation energy of methane adsorption on a reduced Ni-25 cluster was calculated at 108 kJ/mol, while for various Ni25O clusters (modelling partially oxidized surfaces) the activation energies were significantly higher. (c) 2006 Elsevier B.V. All rights reserved.