The use of the spline function, logarithmic polynomial, Szyszkowski and Temkin equations to estimate the extractant interfacial concentration are discussed. The distribution of the experimental data and its nonlinearity upon the approximation errors and the surface concentration are presented. The error of the interfacial tension isotherm estimation depends upon the equation considered. The Temkin isotherm deviates significantly from the experimental data. The distribution of experimental data affects both the matching of the interfacial tension isotherm with the logarithmic polynomial and the surface excess isotherm. The best approximations (low approximation errors and lack of sensitivity upon experimental data distribution) are obtained with the use of the Szyszkowski equation and the spline function.