The normal-incidence reflectance measurement was employed to obtain the free exciton transition energy (EFX) of AlGaN alloys in AlxGa1-xN/GaN/sapphire heterostructure grown by metalorganic chemical vapor deposition. It was found that the thickness variation of the AlGaN layer may cause a noticeable change in the line shape of reflectance spectrum and impede the identification of the desired excitonic position. By using a reflection model of two absorbing layers with a transparent substrate, the experimental reflectance spectra were theoretically simulated and utilized to explain the reflection mechanism in AlxGa1-xN/GaN heterostructures. On the basis of the above analysis, the feasibility of the reflectance measurement for such heterostructures is confirmed. At room temperature, the EFXs obtained from the fitting showed an excellent agreement with the corresponding peak energies in the photoluminescence spectra. Furthermore, at the optical energy position about 100 meV above the EFX, the spectral feature of exciton-LO phonon interaction was observed in the reflectance spectrum record for low Al composition (x≤0.16). Using the Al mole fraction derived from x-ray diffraction measurement, the bowing parameter of the epitaxial AlGaN layer was determined. In the range of 0≤x≪0.3, the resulting bowing parameter shows a downward value of 0.53 eV. © 2001 American Institute of Physics.