Experimental measurements of the cation vacancy formation energies (Ef) of Hg1−xCdxTe were carried out by Hg‐annealing and rapid quenching to room temperature, followed by Hall measurements at 77 K. Our observations show that one charge state vacancy is dominant, so that the fractional numbernof cation vacancies at temperatureTis related exponentially to the energyEfrequired to create one vacancy,n=A exp(−Ef/kT). No appreciable temperature dependence ofEfdue to temperature variation of the ionization levels and the self‐consistent Fermi level is seen. Our measured value for the activation energy is found to beEf=0.9±0.1 eV, for Hg1−xCdxTe withx=0.21, 0.3, and 0.43. This value is found to be nearly independent of Cd compositionx. Theoretical calculations of thep‐type carrier concentrations due to cation vacancies in Hg0.8Cd0.2Te yield results which are in good agreement with experimental data. These modeling calculations predict the cation vacancies to be predominantly doubly ionized species at 77 K.