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Optical energy gaps are measured for high-quality Al 1− x In x N-on-GaN epilayers with a range of compositions around the lattice match point using photoluminescence and photoluminescence excitation spectroscopy. These data are combined with structural data to determine the compositional dependence of emission and absorption energies. The trend indicates a very large bowing parameter of≈ 6 eV and differences with earlier reports are discussed. Very large Stokes’ shifts of 0.4–0.8 eV are observed in the composition range 0.13< x< 0.24⁠, increasing approximately linearly with InN fraction despite the change of sign of the piezoelectric field.