Binodal and spinodal isotherms in a MgxZn1-xSySe1-y system are calculated in a regular solution approximation. Instability and immiscibility regions at typical growth temperatures are isolated from the sides of a composition square diagram and cover a wide range of compositions. The spinodal decomposition of MgxZn1-xSySe1-y alloys occurs along the MgS-ZnSe diagonal, i.e. practically along the line of compositions lattice matched to GaAs. The phase separation does not follow the increase in the strain energy of the system. The spinodal and binodal isotherms calculated with and without taking account of strain effects are practically identical. The results of calculation are in good agreement with available experimental data.