The refractive index and extinction coefficient of cubic GaN in the energy range of 1.5–3.7 eV were determined with high accuracy using combined reflectivity and spectroscopic ellipsometry studies of layers grown by molecular beam epitaxy on GaAs(001). A comparison of the experimental reflectivity data with theoretical calculations demonstrates that the data analysis has to be performed by taking into account both surface roughness and a nonabrupt substrate–film interface. In the transparent region the refractive index of cubic GaN was found to be slightly higher than that of the hexagonal modification. © 1999 American Institute of Physics.