XPS analysis was used to characterise GaAs, Al0.4Ga0.6As and Al0.8Ga0.2As targets bombarded with an O2+ beam ranging from 1 keV to 5 keV. The bombardments were carried out at different angles of incidence from 0° to 60°. In all cases a strong preferential sputtering of arsenic gives rise to an As-depleted altered layer, formed by a mixture of gallium, arsenic and aluminium oxides with metallic gallium and arsenic. Aluminium and gallium are preferentially oxidized, a fact that correlates with the corresponding heat of formation of the oxides, whereas the arsenic oxidation requires a minimum incorporation of oxygen into the matrix. For all experimental conditions a considerably lower oxidation level has been observed in every case as compared to the IBO of silicon.