Thin films of wide-band zinc sulfide ZnS (Eg ~ 3.68 eV) find practical application in optoelectronic devices, solar cells, lasers, are used as base layers of thin-film electroluminescent devices. Doping zinc sulphide with various metal ions affects the change in structural and optoelectronic properties. Of particular interest are Mn x Zn1- x S substitution solid solutions, which have a different band gap than zinc sulphide, allowing their application field to be expanded. Among existing techniques for thin films, the simplest in technological terms, not requiring expensive equipment because of the relatively low temperature synthesis is the method of chemical deposition from aqueous media. It makes it possible to obtain solid solutions of a wider range of compositions. In order to find optimum conditions for producing Mn x Zn1- x S solid solutions, the analysis of ionic equilibria in the reaction system «Zn2+ - Mn2+ - NH3 - OH- - N2H4CS» was carried out beforehand. Thermodynamic evaluation presented in coordinates «initial concentration of Zn (Mn) salt - pH of medium - concentration of NH3 ligand» and «initial concentration of Zn (Mn) salt - pH of medium - concentration of N2H4CS chalcogenizer» allowed to determine potential region of ZnS and MnS joint formation. It is shown that the probability of formation of Mn x Zn1- x S substitution solid solutions exists in the pH range from 11 to 13. At the same time, conditions for the deposition of the accompanying phases - zinc and manganese hydroxides - were found. Preliminary experiments established the formulation of reaction mixtures for chemical coprecipitation of zinc and manganese sulfides, which made it possible to obtain MnZnS films. The thickness of thin-film layers obtained at 353 K for 120 min on quartz substrates decreased from 190 to 140 nm with a simultaneous decrease in the fraction of nanoparticles from ~13% to their complete absence with an increase in the concentration from 0.0005 to 0.01 mol/L MnCl2 in the reactor.