Surface and colloidal properties of local anesthetics were studied by measuring the surface tension, density, differential scanning calorimetry, light transmittance, and electrode potential of aqueous anesthetic solutions. By applying thermodynamic equations to the physicochemical data, the surface and colloidal behavior of local anesthetics in the aqueous solution was characterized. Local anesthetics having strong hydrophobicity such as dibucaine and tetracaine showed micelle formation, phase transition in surface adsorbed films, and Krafft phenomenon. Further, the effect of counterions on the above phenomena was clarified on the basis of the calculated thermodynamic quantities. The magnitude of hydrophobicity of local anesthetic is closely related to the potency of the anesthetic action. In other words, the pharmacological effects of local anesthetics are proportional to the hydrophobicity of the molecules.