The interactions of H and H-2 with W(1 0 0)-c(2 x 2)Cu and W(1 0 0) have been investigated through density functional theory (DFT) calculations to elucidate the effect of Cu atoms on the reactivity of the alloy. Cu atoms do not alter the attraction towards top-W sites felt by H-2 molecules approaching the W(1 0 0) surface but make dissociation more difficult due to the rise of late activation barriers. This is mainly due to the strong decrease in the stability of the atomic adsorbed state on bridge sites, the most favourable ones for H adsorption on W(1 0 0). Still, our results show unambiguously that H-2 dissociative adsorption on perfect terraces of the W(1 0 0)-c(2 x 2)Cu surface is a non-activated process which is consistent with the high sticking probability found in molecular beam experiments at low energies. (C) 2007 Elsevier B.V. All rights reserved.