The electroless dissolution of p-GaAs in aerated 0.5 M boric acid solutions, in the absence and in the presence of Co(II) ions in the solution, is reported. At the pH of these solutions, the anodic dissolution of GaAs occurs in oxide-forming conditions. The oxide film formed during GaAs dissolution is composed of Ga2O3. The evolutions of the morphology and of the thickness of the oxide layer are followed using scanning electron microscopy (SEM), energy-dispersive X-ray analyses (EDX), and X-ray photoelectron spectroscopy (XPS). All of the results show that the oxidation rate of p-GaAs in an aerated boric acid solution is enhanced in the presence of Co(II) in the solution; the increase is larger for higher Co(II) concentration. This leads to an increase of the oxide layer thickness. From the position of the redox potentials of the various species present in the electrolyte, with respect to the GaAs valence bandedge, it appears that dioxygen is responsible for the GaAs anodic dissolution. At the GaAs/solution interface, O-2 reduction occurs, leading to GaAs oxidation. The mechanism of O-2 reduction at the GaAs surface is discussed regarding the energetic situation at the interface. The influence of the Co(II) on the kinetics of O-2 reduction is discussed, taking into account the results of the literature.