"Spectroscopic ellipsometry measurements of InXGa1-XN thin films were carried out in the photon energy range from 0.6 to 4.75 eV. The samples were grown on cubic GaN/MgO (100) template substrates by plasma assisted molecular beam epitaxy. Optical properties as the energy gap, refractive index (η) and extinction coefficient (κ) were obtained from the analysis of experimental data by a parametric dielectric function model. Our results show that the behavior of the optical band gap of cubic InXGa1-XN fits Eg(x) = 1.407x2 − 3.662x + 3.2 eV. The obtained bowing parameter of 1.4 ± 0.1 eV is in good agreement with reported calculated values around 1.37 eV. The complex index of refraction dispersion relations η(ω) and κ(ω) are obtained for the 85–99% mostly cubic InXGa1-XN films for several In concentrations."