The electrical characteristics of anodic oxides on sulfur-passivated InP were investigated. The effects of passivation of InP and subsequent rapid thermal annealing on the oxide included reduction in the fixed charge density, and increases in both breakdown voltage and resistivity. The permittivity and conductivity of the oxide were also influenced by the passivation. A model based on the changes in the structure of the oxide is proposed to explain the results. According to this model, a reaction takes place between sulfur atoms and the oxygen atoms of In2O3 which results in an increase in the P2O5/In2O3 ratio and modification of the oxide structure.