Infrared transmission and photoconductivity spectra of mercury cadmium telluride (MCT) epitaxial layers, grown by liquid phase epitaxy (LPE) on CdTe and CdZnTe wide band gap substrates, were investigated both theoretically and experimentally at temperatures T =82 K and T =300 K in the infrared (IR) wavelength region 3 15 μm. The photoresponse position of the diodes was determined at cryogenic temperatures from the transmission spectra of room temperature. Theoretical calculations of optical density D(h-ω), needed for analysis of experimental optical transmission data, were performed in the framework of WKB approximation.