The initial stages of anodization of GaAs and InP were investigated in order to determine the factors which affect the oxide–semiconductor interface formation. This was accomplished by ESCA profiling thin anodic layers grown on chemomechanically polished wafers. Oxides on both InP and GaAs appear to be initiated by a nucleation and island growth process with the film becoming continuous at a thickness of approximately 40 Å. The oxide/GaAs interface was found to be composed of a Ga2O3 layer with indications of elemental As near the substrate. The oxide/InP interface had a uniform distribution of In2O3 and P2O5. The data indicates that dissolution of As2O3 or P2O5 is not a significant factor in the interface formation but the thermodynamic stability of the oxide could be important.