The role of Gd2O3 is investigated in our previously discovered oxide films of Ga2O3(Gd2O3) for GaAs surface passivation. Based on the systematic dependence of the dielectric properties of (Ga2O3)(1-x)(Gd2O3)(x) on the Gd (x) content, we showed that pure gallium oxide does not effectively passivate GaAs, and Gd2O3 is a necessary component to stabilize the gallium oxide in the 3(+) fully oxidized state due to the electropositive nature of Gd+3. This gives rise to electrically insulting films of low leakage current and high electrical breakdown strength.