The results of an ab initio all-electron Hartree-Fock study of the interaction of oxygen with Al10 and Al3 clusters are reported. These clusters model the close-packed Al(111) surface. The Al10 cluster has seven atoms in the first layer and three in the second; the Al3 cluster has only three atoms in the first layer. The chemisorption of atomic oxygen at the f.c.c. sites of Al(lll) has been studied by means of calculations on A1I0O3, Al10O and A13O. The Al10O3 cluster was chosen to provide the opportunity of incorporating part of the possible influence of oxygen-oxygen interactions on the chemisorption properties. A stable phase of oxygen has been found to be chemisorbed just outside the surface, with a vertical distance between oxygen and the first Al plane of - 0 6 Å. The calculated vibrational energy of the oxygen motion in a direction normal to the surface in the harmonic approximation is - 50 meV. The results are consistent with the coexistence of oxygen in overlayer and underlayer configurations. No significant influence of oxygen-oxygen interaction on the calculated properties has been found. Finally, the calculation of the chemical shift of surface Al (2p) ionization is discussed. © 1985 Taylor & Francis Ltd.