The present work is devoted to the thermodynamic analysis of physicochemical processes of growth of epitaxial layers of semiconductor compound Ga2Se3 from the gas phase in the flow system Ga - Se - Cl - H and modeling of technological processes for forecasting of possible technological variants and determination of optimal conditions for the synthesis of this compound. The results of researches on definition and calculation of thermodynamic parameters of individual substances of Ga - Se - Cl - H system are given, for their use at thermodynamic modeling of processes of growth of epitaxial layers of Ga2Se3 from a gas phase. The connection between thermodynamic variables and technological parameters of Ga2Se3 synthesis process in open-type reactor with separate sources of gallium and selenium is investigated. The scheme of calculation of technological parameters of the process of growing epitaxial layers of Ga2Se3 in the flow gas transportation system with separate sources of gallium and selenium is illustrated.