We report on photon-induced modifications in CdTe/MCT (cadmium telluride/mercury cadmium telluride) heterostructure interfaces formed by flash evaporated CdTe on to the bulk p-Hg0.8Cd0.2Te. Metal–insulator–semiconductor (MIS) test structures were processed and their electrical properties measured by capacitance–voltage (C–V) and current–voltage characteristics. The effect of preannealing the MCT wafers under photon excitation and in Hg environment as well as the postannealing of CdTe/MCT under photon excitation were investigated. By optimizing the preanneal and postanneal processes, a significant improvement in the interface characteristics was obtained as evidenced by C–V measurements. The ultraviolet (UV) photons seem to play an important role in the process. The Hg atoms in the vapor phase absorb UV photons. In the presence of excited Hg atoms, hydrogen radicals are formed by direct collisions in the vapor phase and passivate the MCT surface.