The effects of rapid thermal annealing (RTA) and sulfur passivation of GaAs on the electrical properties of GaAs metal oxide semiconductor (MOS) structures were investigated. The oxide layers were grown by anodization. Annealing was performed after oxidation and before aluminum evaporation. Passivation improved the electrical properties. Further improvements were obtained after RTA. The MOS structures on passivated GaAs showed good thermal stability compared with those on unpassivated GaAs. The bond model and possible modifications in the structure of the oxide were used to explain the observations.