In addition of the electrical properties (resistivity, charge carrier mobilities), the optical properties are important parameters of a transparent conductive oxide (TCO) and of a transparent amorphous oxide (AOS). The transparency spectral range, the refractive index dispersion, as well as the absorption coefficient dispersion (or extinction coefficient) define the optical properties, and can be used for example to estimate the amount of light that reaches the core p-n junction of a photovoltaic cell. Inx−wGawZn1−xO1+0.5x−1, known as IGZO or GIZO, have been proposed as a suitable thin film transistor’s channel semiconductor. Such amorphous oxide semiconductors (AOS) can be used on already developed silicon based platforms or on novel proposed fully transparent devices, while IZO can replace as TCO the most expensive Sn:In2O3 (ITO). Due to the different structure in which ZnO (hexagonal wurzite), In2O3 (cubic bixibite) and Ga2O3 (monoclinic) crystallize, the intermixture of these compounds has for a large concentration range an amorphous-like structure, whereas the electric properties can be tailored by modifying the stoichiometry. This amorphous nature is a big advantage versus pure ZnO which has a strong tendency to form polycrystalline phase, which diminishes the electrical and bending functionality. The optical properties of the amorphous IZO and IGZO films having various In and Ga concentrations were determined by analysis of spectroscopic ellipsometry data. The refractive index of non-absorbing materials (k=α=0) in the corresponding transparency spectrum is linearly dependent on the mass density, determined by X-Ray Reflectivity. However, this dependence between the refractive index and the mass density breaks down in the case of a weak absorbing compound. At certain cations concentration, a semiconductor-metal transition occurred, as proved by the free carrier light absorption and by the specific band-gap blue shift.