Solid solutions in the system zinc sulfide/zinc phosphide (Zn2+xS2-2xP2x) were investigated using the cyclic cluster model within the semiempirical MSINDO method. Results of cyclic cluster calculations for binding energies of the perfect ZnS and Zn3P2 are presented and compared with the experimental data. The miscibility of ZnS and Zn3P2 over the whole composition range of 0 < x < 1 was investigated by calculating the Gibbs free energy of mixing Delta(M)G for different values of x. A miscibility gap was found at both ends of the composition range and compared with experimental data.