Ellipsometry is a sensitive nondestructive analytical technique well suited to the molecular‐beam epitaxy (MBE) growth process. We report its use in real time to control composition, growth rates, interdiffusion, and growth related surface defects of Cd0.2Hg0.8Te epilayers and CdTe/HgTe superlattices grown on CdTe and Cd0.96Zn0.04Te (100) substrates using a Riber 32 R&D MBE machine that has been custom modified by the manufacturer to facilitate the addition of a phase‐modulated ellipsometer. Growth rate data from ellipsometry are in good agreement with reflection high‐energy electron diffraction intensity oscillation data, double‐crystal rocking curve determinations of superlattice periodicity, and infrared transmission measurements of total thickness. The level of Hg incorporation in CdTe (100) layers is measured with ellipsometry and its temperature dependence is established. Growth‐front roughening of CdTe during the growth of CdTe/HgTe superlattices and interdiffusion between these layers on postgrowth annealing, are observed in the ellipsometric data. The results are compared with model calculations and a compositional profile of these interface structures is proposed.