Abstract <p>The degree of asymmetry of the excess surface tension (σ) isotherms of boundary binary systems was found to be the main criterion for preliminary assessment of the applicability limits of the Kohler method. For quantification of the degree of asymmetry of the excess surface tension isotherms of binary systems, we used characteristics of the distribution of a random variable (asymmetry coefficient, excess). The surface tension and asymmetry coefficients in boundary binary systems were calculated for 12 ternary systems for which reliable experimental data were available. The ternary systems can be divided into several groups according to the average asymmetry coefficient of two boundary binary melts: insignificant asymmetry (the average value of the asymmetry coefficient is smaller than 0.7; the σ values obtained by the Kohler method coincide with the experimental data within the measurement error); moderate asymmetry (the average value lies in the range from 0.7 to 0.8; the σ isotherms qualitatively describe the experimental curves); and significant asymmetry (the average value is greater than 0.8; the theoretical curves do not describe the experimental curves).</p>