Electroreflectance is applied to study the interface properties of metal/SiO2/GaAs structures. From a bias‐voltage dependence of surface potential, which was evaluated from the measured surface field ini/n+‐GaAs structures, energy distributions of interface‐state density were determined based on the modulation spectroscopy technique. Upon biasing toward inversion, a strong pinning of surface Fermi level occurred at around 0.88 eV below the conduction‐band minimum. This pinning, thus, is concluded to be due to a large number of interface states, and not due to an actual hole inversion. The method is free from numerical ambiguities encountered in capacitance‐based techniques, in which the energy reference for surface Fermi level is model dependent, and thus, is a suitable tool for studying insulator/semiconductor interfaces, even having a large number of interface traps. ©1996 American Institute of Physics.