We have used polarized photoreflectance spectroscopy to study the electronic-band-structure modification of GaN films grown along different nonpolar orientations due to biaxial, anisotropic in-plane strain. For nonpolar oriented films, the c-axis of GaN lies in the film plane. An unstrained, high-quality C-plane GaN film is used to estimate the difference in the band gap energies between 10 K and room temperature. We use the crystal-field and spin-orbit splitting energies and the deformation potential D, determined at low temperatures to calculate the transition energies and the polarization properties of nonpolar oriented films at room temperature using the k-p perturbation approach. The calculated transition energies and oscillator strengths are then compared to the experimentally obtained values. (c) 2007 WELEY-VCH Verlag GmbH & Co. KGaA, Weinheim.