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. ©1995 American Institute of Physics.