This work is focused on better understanding of the origin of CO2/CO selectivity of methanol steam reforming (MSR) catalyzed by three isostructural (L1(0)-type) intermetallic compounds NiZn, PdZn, and PtZn. Following earlier suggestions we assume that the selectivity of MSR arises from the competition between two possible transformations of the reaction intermediate formaldehyde: the reaction of formaldehyde with hydroxyl leading ultimately to the formation of CO2, and the direct dehydrogenation of formaldehyde leading to the production of CO. We have investigated whether the differences in the activation energies for these steps explain the experimentally observed trends in selectivity. Using density-functional theory detailed atomistic scenarios for both reactions have been developed. As in vacuum the reaction of formaldehyde with hydroxyl is non-activated, it does not need the support of a catalyst to proceed. There is a clear correlation between high CO2 selectivities and weak adsorption energies of formaldehyde. On the L1(0)(111) surfaces we have to consider the existence of four inequivalent adsorption sites, caused by the monoclinic shift of subsurface atomic planes resulting from tetragonal deformation of the L10 structure. At PtZn(111) this shift could explain the observed mixed CO2/CO selectivity of the Pan catalyst. The proposed mechanism of selectivity can qualitatively explain the experimentally observed selectivities of the investigated intermetallic compounds. Our results can contribute also to understanding the influence of the ZnO support and the Zn-enriched PdZn surface on the selectivity of the MSR process. The possibilities how to improve the CO2 selectivity of the catalyst are also presented. (C) 2015 Elsevier Inc. All rights reserved.