A systematic study of line shapes obtained by electrolyte electroreflectance (EER) and by automatic spectroscopic ellipsometry (ASE) on samples of mercury cadmium telluride (MCT) with varying densities of defects has been carried out. The results show that the two techniques yield in all cases the same value for the interband transition energies Ej. Because the values of the Ej are quantitatively related to the alloy composition x, it is clear that both ASE and EER can be used to carry out studies of the alloy composition. On the other hand, the values of the linewidths Γj yielded by the two techniques often do not agree. In those cases in which the etch pit density is less than 106/cm2, the values agree approximately; otherwise, EER yields much larger values. The difference between the two techniques is, of course, the use of an electric field modulated at very low frequencies (EER). We show that the modulating electric field causes an electrostriction and polarization of the defects in MCT. These effect...