Optical second harmonic generation has been used to study oxide‐nitride‐oxide/Si(111) interfaces. The second harmonic (SH) intensity is measured as a function of an azimuthal rotation angle at a 45° incidence angle. The SH intensities change for wet oxidation, nitridation and reoxidation. The SH intensity from wet oxide/Si(111) interface decreases as the temperature during wet oxidation increases. With subsequent nitridation, this SH intensity of the oxidized interface then decreases. However, the SH intensity increases again with reoxidation. ©1995 American Institute of Physics.