The properties of Ga2O3 films evaporated on unpassivated and passivated GaAs surfaces using a Gd3Ga5O12 source were studied. The properties of unpassivated (passivated) films were a strong (weak) function of substrate temperature with best results obtained at a substrate temperature of about 100 degreesC. The thermal stability of films evaporated at a substrate temperature of 100 degreesC was studied as well. The passivated films showed a better thermal stability. The bonds formed at the surface layer of passivated GaAs were used to explain the results.