Abstract It has been found that two bile salt solutions of NaC (trihydroxy cholanic salt ) and NaDC (dihydroxy cholanic salt have an excess ultrasonic absorption . This excess absorption fits a single relaxation in all cases . To explain this observed single relaxation the data are considered in terms of several existing models. Simple cation-anion reactions to form a monomer salt are ruled out on the grounds that our experimentations are carried out at concentrations far above the critical micelle concentrations (CMC). A second approach in which the excess absorption is attributed to micelle- counter ion reaction. The third existing model deals with an equilibria between the micelles and the monomers of the bile salts. Three different types of models emerge from this kind of equilibria. Only two (3a and 3b ) of them can be considered as a feasible possibility to explain part of the results.