The partitioning of five hydrochloride salts of local anesthetics, dibucaine (DC . HCl), tetracaine (TC . HCl), bupivacaine (BC . HCl), lidocaine (LC . HCl), and procaine (PC . HCl), into surface-adsorbed films and micelles formed by decylammonium chloride (DeAC) was studied by the surface tension of aqueous solutions of DeAC-local anesthetic mixtures. Thermodynamic quantities of the partitioning of the anesthetics, total surface density, and compositions of the anesthetics in the surface-adsorbed film and micelle were evaluated by applying thermodynamic equations to the surface tension data. The quantities of the anesthetics partitioned into the molecular aggregates of DeAC were determined from the phase diagrams of surface adsorption and micelle formation. The phase diagrams of surface adsorption and micelle formation showed that the local anesthetic partitioned into the surface-adsorbed film and micelle of DeAC decreases in the order of DC . HCl, TC . HCl, BC . HCl, LC . HCl, and PC . HCl. A good correlation was seen between the partitioning order and anesthetic potency of these local anesthetics. The phase diagrams of DeAC-BC . HCl, DeAC-LC . HCl and DeAC-PC . HCl systems behaved peculiarly that the compositions of these systems in the adsorbed film and micelle had negative values. The results suggested that weakly hydrophobic anesthetics such as BC . HCl, LC . HCl, and PC . HCl did not partition into the hydrophobic environment of the adsorbed film and micelle of DeAC. Strongly hydrophobic local anesthetics such as DC . HCl and TC . HCl partitioned into the aggregates of DeAC. The difference is attributable to the hydrophobicity of their molecules. By comparing the compositions of micelle with those of surface-adsorbed film at the critical micelle concentration, it was shown that the partitioning of the anesthetics was also influenced by the geometry of the aggregates into which the anesthetics were partitioned.