The and adsorption behaviour of the amphiphilic drug promethazine hydrochloride (PMT - a phenothiazine) and bile salts mixtures were analyzed at different compositions in pure and mixed states in aqueous solutions. By using regular solution theory (RST) and Rosen's model different physicochemical properties such as critical micellar concentration (cmc), micellar composition, surface excess concentration (Gamma(max)), minimum area per molecule (A(min)), interaction parameters (beta(m), beta(sigma)), energetic parameters of micellization as well as other micellar and surface properties have been determined. The values of interaction parameters (beta) and activity coefficients f(1) and f(2) (for both, in mixed monolayer as well as in mixed micelles) indicate the synergistic behaviour. The thermodynamic parameters propose release of water from the hydrophobic portion of the drug at higher temperatures. (C) 2013 Elsevier Ltd. All rights reserved.