In this work. first-principles modeling techniques are used to investigate the mechanism(s) of adsorption of molecular oxygen on the GaAs(0 0 1)-(2 x 4) surface. The reaction of adsorption was modeled using ab-initio molecular dynamics at constant temperature for two thermal regimes, i.e. 300 K and 680 K respectively. The resulting adsorbate configurations were relaxed using density functional theory and the adsorption energies were Subsequently computed. Our results suggest a dominant mechanism of adsorption described by molecular dissociation, followed by oxygen insertion in the Ga-As bonds, bridging Ga-O-As chemical bonds. The electronic properties of the clean reconstructed GaAs(0 0 1) Surface and the ones obtained after O-2 adsorption were computed. It is found that for the most stable adsorbate configuration, where oxygen is incorporated in a Ga-O-As unit, the associated density of electronic states is free of defect levels within the GaAs band gap region. (C) 2008 Elsevier B.V. All rights reserved.