The incorporation of two micelle-forming local anesthetics, dibucaine hydrochloride (DC . HCl) and tetracaine hydrochloride (TC . HCl), into surface-adsorbed films and micelles formed by decylammonium chloride (DeAC) was studied by measuring the surface tension of aqueous solutions of DeAC-DC . HCl and DeAC-TC . HCl binary mixtures. The quantities of the anesthetics incorporated into the adsorbed film and micelle of DeAC were estimated from the compositions of the anesthetics in the adsorbed film and micelle, which were obtained by thermodynamic analysis of surface tension data. The phase diagrams of surface adsorption and micelle formation constructed on the basis of composition in the molecular aggregates indicated that although significant amounts of both anesthetics were incorporated into the adsorbed film and micelle, larger quantities of DC . HCl than of TC . HCl were present in the hydrophobic environment of the aggregates. The relative ease with which these two anesthetics were incorporated into the model membranes correlates with their observed clinical potency. Interactions between DeAC and the anesthetic molecules in the adsorbed and micellar states were also analyzed by calculating the ideal mixing lines in both states. It was shown that the bulky polar head groups of the anesthetics were more favorable for micelle formation than for surface adsorption by comparing the incorporation of the anesthetics into the adsorbed film with that into the micelle at the critical micelle concentration.