Proportion concentrations mononuclear hydroxocomplexes of gallium(III) depending on pH was determined by calculating the ionic equilibria using thermodynamic constants. Found that hydrolysis of metal ions in the system "Ga(NO3)3 - H2O" begins at pH ≥ 1.5, whereas in the system "Ga(NO3)3 - Na-EDTA - H2O" at pH ≥ 9. The boundary conditions of formation in systems “Ga(NO3)3 - H2O - N2H4CSe” and “Ga(NO3)3 - H2O - Na2SeSO3” and concentration deposition plane “Ga(NO3)3 - Na-EDTA - H2O - N2H4CSe” and “Ga(NO3)3 - Na-EDTA - H2O - Na2SeSO3” were calculated for the solid phases of selenide and hydroxide of gallium(III).