Mechanism of the associative desorption of oxygen from the Pt( 1 1 1) surface has been studied on atomic level by means of density-functional calculations. Key to the association of two oxygen adatoms into the O-2 molecule is the excitation of one of the adatoms to on-top site, where it becomes essentially neutral. The related redistribution of the electronic density at the O adatom leads to the appearance of the lateral attraction with the other O atom, thus providing an efficient channel for associative desorption. Calculated local densities of states resemble the transformation of the electronic structure of adsorbed O adatoms from the reactive to bound state in the course of association. (c) 2008 Elsevier B. V. All rights reserved.