The surface polarity of a mercury cadmium telluride (MCT) (111) crystal surface has been determined by x‐ray photoelectron diffraction (XPD). Emission from the core levels of Hg, Cd, and Te gave reproducible photoelectron diffraction patterns with considerable fine structure. Comparisons between experiment and single scattering cluster calculations viaRfactors very well distinguished the different kinds of lattice sites of Cd, Hg, and Te, and also permitted unambiguously assigning a cationic termination to the sample studied. This is thus a demonstration of the capability of XPD to study the type of termination involved at MCT and other compound semiconductor surfaces.