Salts of bile acids are steroid amphiphilic compounds which form micelles consisting of 2-12 monomer units. In nature, the formation of mixed micelles with phospholipids (gall bladder), increases the capacity of bile salts to solubilize cholesterol. Oxo derivatives of bile salts are of special research interest because Of their lower membrane toxicity compared to the corresponding OH analogues. In view of the fact that the micelles of anions of oxo derivatives have lower hydrophobicities, it was interesting to study mixed micelles of bile salts and the hydrophobic nonionic cosurfactant Tween 40. With this aim, the critical micelle concentrations were determined by conductometric and tensiometric methods. It was found that the absolute value of the interaction parameter (measure of the synergism between different constitutive units, i.e., the measure of the departure from an ideal micelle) increases with the increase in the number of equatorial OH or oxo groups in the steroid skeleton of the bile acid anion. Namely, the oxygen atoms of these groups are shifted toward the steroid skeleton mean plane, which facilitates effective formation of hydrogen bonds with the polyoxyethylene chains of the polar head.