The quasi-ternary system (In2(1-y-z)Ga2yCd3zV(1-z))Te-3 is characterized by the fact that at high temperatures, all three binary compounds crystallize in the zincblende lattice. The solid solutions in this system contain structural vacancies V. Near special stoichiometric compositions this vacancies cause ordering processes in the metal sublattice. The occurrence of both ordering and demixing within one and the same quasi-ternary system can be described by a single excess Gibbs energy function. The tendency to demixing is considered by interaction parameters derived from the quasi-binary edge systems, the compound formation by adding ordering contributions to the Gibbs energy in form of Gaussian distribution functions. Experimentally, the phase diagrams were determined by X-ray analysis, by electron microprobe analysis and by DTA.