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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 D5 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. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)