The index of Cd1−xMgxTe ternary alloys was measured for the first time by variable-angle spectroscopic ellipsometry on layers of different concentrations. The ellipsometer’s wide spectral range (0.7–5.6 eV) clearly reveals critical points beyond the gap. Self-consistency of the permittivity measurements is investigated by Kramers–Kronig analysis. A transition layer is revealed at the top surface of the samples and is taken into account as a rough layer. In the transparent region Sellmeier’s law is applied to describe the index behavior as a function of the wavelength and the magnesium content.