In quasiternary systems composed of II-VI and III-VI compounds the structural vacancies determine the interdiffusion within the cation sublattice. Interdiffusion coefficients for quasibinary sections through the phase diagrams are calculated by the Boltzmann-Matano method from X-ray intensity profiles taken with an electron microprobe, In substances of those regions of the phase diagram, where the sites in the cation sublattice are randomly occupied, the structural vacancies act as point defects favouring the interdiffusion of Cd and Hg or In and Ga. In structurally ordered phases, however, the vacancies act as regular interstitial sites, Moreover, an influence of the reduced temperature T/T-F on the interdiffusion could be detected.