In this paper author has elaborated a model which provides a process of supersaturated solid solution hydrochemical synthesis in the PbSe – SnSe system by co-precipitation of metal selenides. It was shown that probability of supersaturated solid solution formation Pb 1-xSn xSe with SnSe content up to 13.8 mol. % is existing. The computed results of film hydrochemical synthesis Pb 1-xSn xSe harmonize with experimental data.