We have carried out a systematic study of mercury cadmium telluride (MCT), bulk single crystal and epilayers, using electrolyte electroreflectance (EER). Our results, and others discussed here, suggest that much of the unusual properties of this material are due to structural microdomains. Under mechanical stresses or chemical attack, the walls of these domains tend to move via the generation and propagation of defects associated with cationic diffusion. A typical consequence is the creation of n-type zones within a p-type matrix. We have calibrated our etch rate so as to be able to apply EER to the study of the profiles of epilayers with a depth resolution of 200 Å. The technique was applied to more than 100 samples of all origins. The results are summarized here in qualitative terms, such as typical profile features, as well as in quantitative terms, such as compositional variations and defect densities. EER results are integrated within the conceptual framework introduced above and compared to SIMS and microprobe results.