The free concentrations of local anesthetic cations in equilibrium with sodium dodecyl sulfate (SDS) micelle which solubilized the anesthetic were determined by using ion-selective electrodes sensitive to local anesthetics, procaine (PC), lidocaine (LC), and mepivacaine (MC). Solubilizate distribution between water and SDS micelle was analyzed by means of the stepwise mass-action model. Association constant, K(1), was found to depend upon the anesthetic concentration, which decreased exponentially as the concentration of free anesthetic increased. Therefore, K(1) should include the interaction function φ(A) as K(1)=K(int)exp{-φ(A)} where K(int) is an intrinsic association constant. φ(A) is an increasing function of the anesthetic concentration, which means that occupation of a solubilization site by a local anesthetic cation makes sequential solubilization more difficult. The binding affinity of an anesthetic with SDS micelle increased in the following order PC<LC<MC.The critical micelle concentration (CMC) of mixed micelle was determined as a function of the concentration of free anesthetic. The CMC decreased with an increasing amount of anesthetics solubilized. All the anesthetic compositions in the micelle calculated thermodynamically from the CMC data were larger than the corresponding ones in the aqueous phase. Although the local anesthetics used here do not form micelles by themselves, the CMC vs composition curve can be regarded as a part of a micellar phase diagram showing the negative azeotropic behavior, which reflects the attractive interaction between the anionic surfactant micelle and the local anesthetic cation.