Valence band x-ray photoelectron spectra (XPS) from oxide films grown on GaAs surfaces by thermal oxidation and electron cyclotron resonance (ECR) plasma oxidation are compared. The present work along with previous studies clearly show that the ECR grown oxides are nearly stoichiometric and close toGaAsO4while thermally grown oxides are closer toGa2O3having significant amounts ofAs+3oxidation states. Metal oxide semiconductor (MOS) structures were prepared by coating both the thermal and ECR plasma grown GaAs oxides with ECR plasma enhanced chemically vapor deposited (PECVD)SiO2and then with evaporated Al as top contact. Interface charges in terms of fixed oxide charge and surface electronic states were obtained from conventional capacitance–voltage measurements. Despite the significant difference in the GaAs oxides structure, most notably theGaAsO4like ECR plasma oxides, the interfaces prepared by both thermal and plasma oxidation were found to be equally unpassivated in terms of high levels of interface electronic states and Fermi level pinning.