In this work mixed micelles of Na salts of bile acids and Tween 80 are studied. In the experiment, number and orientation of OH and oxo groups in the steroid skeleton of bile acid anion varies. In every investigated mixture, mole fraction of Tween 80 (alpha) varies from 0.05 to 0.5 of the total surfactant concentration. Critical micelle concentrations of binary surfactant mixtures are determined by and conductometric methods (modified Philips method of data processing). By employing regular solution theory for every studied binary surfactant mixture, values of interaction coefficient (beta(1,2)) are determined. Based on the obtained beta(1,2) values, investigated bile acid anions form two groups. Group I consists of bile acids having only OH groups attached to the steroid skeleton. Characteristics of mixed micelles of this group are negative values of beta(1,2) for a to 0.1 (synergistic effect between different types of the micelle building units), and positive values of beta(1,2) for a over 0.1 (antagonistic effect). This change of sign of beta(1,2) as a function of alpha can be explained by phase transformation in mixed micelle as a pseudophase, and due to rigid geometry of cis-oleic residue of Tween 80. Group II is formed of bile acid anions having one or more oxo groups in the steroid skeleton. Mixed micelles of Tween 80 and group II of bile acid anions are characterized by positive beta(1,2) values in the whole investigated range of a. Based on the intensity measurements at different temperatures of first (I-1) and third (I-3) vibrational bands of pyrene (probe molecule) emission spectrum as a function of total surfactant concentration, it can be concluded that mixed micelles having Tween 80 are less rigid than mixed micelles having Tween 40. This also indicates that in hydrophobic domain of mixed micelles, cis-oleic residues are more difficult to pack together compared to the palmitic residues of Tween 40. (c) 2013 Elsevier Ltd. All rights reserved.