The authors report room temperature reflectance measurements on wurtzite C-plane GaN films grown on sapphire substrates. The spectra clearly show exciton related features near the fundamental bandgap of GaN. The Lorentz oscillator model was used to analyze the excitonic contribution to the spectral lineshape and to determine the transition energies. The results are explained by comparison with electronic band structure calculations which include the effect of strain. The analysis gives an independent estimate of the residual biaxial strain in the films. © 2007 IEEE.