The authors present results of a perturbation theory study of the effect of strain on the oscillator strengths of interband transitions in wurtzite group-III-nitride films suitable for ultraviolet light emission applications. Ternary alloy films are investigated, which can be pseudomorphically grown on GaN and AlN substrates with nonpolar M-plane (1 100) and A-plane (1120) orientations. Valence band mixing, induced by the anisotropic in-plane strain that arises in these films, can have a dramatic influence on the optical polarization properties of the transitions. An increased efficiency of light emission in the 0.21 μm to 0.25 μm spectral range is best achieved using AlN substrates, on which both Al1-xGaxN and Al1-xInxN films experience compressive strain. On GaN substrates, Al1-xGaxN films experience tensile strain and will exhibit poor light emission efficiency. However Al1-xInxN films on GaN substrates can emit light efficiently in the 0.3 μm to 0.36 μm spectral range. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.