The growth rate of GaAs anodic oxide in radio-frequency plasma at low temperatures can be strongly affected by an ultrathin aluminum layer ( < 3 nm) evaporated on the semiconductor before oxidation. The effect of this layer on the properties of the resulting oxide-GaAs interface has been investigated. Comparing the structures grown by means of the Al overlayer to the sample with a pure native oxide, the corresponding deep level transient spectra differ markedly and the GaAs lattice contraction below the double oxide is reduced. Application of a fitting procedure to a simple layer growth model leads to interpretation of the investigated process by increasing both the migration coefficient of the negative oxygen ions in the oxide and concentration of O- ions on the dielectric surface during the oxidation procedure.