Regular solution theory (RST) was compared with model-independent thermodynamical methods (MIM) to analyze binary mixed micelles. It was found that if the molar fractions of the surfactants obtained using RST and MIM do not differ, the binary mixed micelles are symmetrical, according to the excess Gibbs energy. The algorithm used to analyze the binary surfactant mixtures, i.e., the method for determining whether the binary mixture of the surfactants is symmetrical and obeys RST, was developed. Binary mixtures of Threen 40 - Na-3,12-dioxo-5 beta-cholanate (T40-3,12-DOC) and Tween 80 - Na-3,12-dioxo-5 beta-cholanate (T80-3,12-DOC) were analyzed. Mixed micelles of T40-3,12-DOC were found to be more stable compared with the hypothetical ideal mixed micelles, i.e., they are primarily symmetrical, according to the excess Gibbs energy, but they do not obey RST. Mixed micelles of T80-3,12-DOC were non-symmetrical, according to the excess free energy, and were less stable then the ideal mixed micelles. With the increase in the molar fraction of T80 in the mixed micelle, the micelle became less stable and decomposed into micelles that comprised the building units. (C) 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.