A quantitative approach to the determination of the ratio between binary and ternary intermetallic phases in the Al—Mg(Cu)—Li alloys is developed on the basis of the balance equations of the chemical and phase compositions as well as the experimentally measured lattice parameter of the $\alpha$-solid solution. As shown, for the Al—Mg(Cu)—Li alloys, the ratio between the fractions of the $\delta^{\prime}$ (Al$_{3}$Li) and S$_{1}$ (T$_{1}$) phases is determined by the ratio between the molar fractions of Li and Mg (Cu). The equations for the calculation of the contents of the S$_{1}$ (Al$_{2}$MgLi), T$_{1}$ (Al$_{2}$CuLi) and $\delta^{\prime}$ (Al$_{3}$Li) phases in the 1420, 1424, 5090 alloys (Al—Mg—Li alloys) and in the 1440, 1460, 1461, 1441, 1469, 2090, 2094, 2095, 8090, Weldalite 049 alloys (Al—Cu—Li alloys) used in Russia and other countries are given. The possibilities of the method application for the study and prediction of the phase stability and anisotropy of the elastic and strength properties are considered.