The optical properties of a-plane Al1−xInxN grown by metal-organic vapor phase epitaxy on an a-plane GaN/r-plane sapphire template are reported. X-ray diffraction yielded an In content of ∼20%. The ordinary and extraordinary dielectric functions (DFs) were obtained by spectroscopic ellipsometry in the spectral range from 1 eV up to 6 eV at room temperature. By fitting the experimentally obtained complex DF, the inter-band transitions EA and EB, which are allowed for configurations and , respectively, were determined. A redshift of ∼200 meV is found for the transition EB with respect to transition EA attributed to the optical selection rules. Furthermore, the ordinary and extraordinary refractive indices in the transparent region (below the band gap) are represented in the analytical form. The AlInN alloy shows the positive birefringence and the difference between the extraordinary and ordinary refractive index ne − no is defined to be 0.068 at photon energy of 3 eV. Finally, the ordinary and extraordinary high-energy dielectric constants were estimated.