Undoped GaAs epilayers grown by metalorganic chemical vapor deposition on Zn‐ and Si‐doped GaAs substrates were studied using the technique of photoreflectance (PR) spectroscopy. Oscillations were observed in the PR spectra of the samples grown onn‐type substrates below the fundamental band gap of GaAs. The origin of these oscillations was investigated in terms of interference between the light beam reflected from the air/film interface and the modulated beam reflected from the film/substrate interface. The latter is due to the modulation of the index of refraction of the substrate. The thickness of each film was calculated by a model which was used to fit the experimental spectra of the differential change in reflectance (&Dgr;R/R) versus energy. The observed changes in the period of these oscillations as a function of film thickness were in good agreement with the calculated results. Scanning electron microscope measurements of the thicknesses of the films supported these calculations.