Incorporation of the local anesthetic procaine hydrochloride (PC . HCl) into surface-adsorbed films and micelles formed by decylammonium chloride (DeAC) was investigated by measuring the surface tensions of aqueous solutions of a DeAC-PC . HCl mixture. The thermodynamic quantities of the miscibility in these molecular aggregates, the total surface density, and the compositions of PC . HCl in the surface-adsorbed film and micelle were calculated by applying thermodynamic equations to the surface tension data. The phase diagrams of surface adsorption and micelle formation showed a peculiar feature; namely, the compositions of PC . HCl in the adsorbed film and micelle showed negative values over the whole composition range measured. The negative values were caused by the exclusion of the PC . HCl molecules from the adsorbed film and the micelle of DeAC. PC . HCl was not incorporated into such hydrophobic environments. In contrast, tetracaine hydrochloride (TC . HCl) was incorporated into the adsorbed film and micelle of DeAC at all DeAC-TC . HCl compositions. The difference in incorporation between TC . HCl and PC . HCl is attributable to the hydrophobicity of their molecules.