The presence of organic molecules in ionic surfactant micellar solutions generally decreases counterion binding. By means of 23Na nuclear magnetic resonance spectrometry, however, Yoshida et al. ( J. Colloid Interface Sci. 109, 336 (1986)) demonstrated that an inhalation anesthetic, halothane, increased the Na + ion binding to anionic surfactant micelles. The result contradicts the previous conductometry study (Kaneshina et al. J. Colloid Interface Sci. 83, 589 (1981)) that showed a slight decrease in the counterion binding to anionic surfactant micelles by inhalation anesthetics. The present study was undertaken to resolve this discrepancy, and the electrical conductivity study was repeated. Light scattering and refractometry showed that an inhalation anesthetic, halothane, did not affect the surfactant aggregation number appreciably under the present experimental conditions. When the anesthetic concentration in the micellar solution was maintained at a constant during the change in the surfactant concentration, the conductivity data indicated a decrease in the counterion binding. However, in this experimental design, the anesthetic concentration in the micelles decreases with the increase in the surfactant concentration. Hence, the anesthetic effect diminishes. When the anesthetic/surfactant mole ratio in the micelle was maintained constant while varying the surfactant concentration, the conductivity data indicated an increase in the counterion binding. Inhalation anesthetics appear to be exceptional among organic solvents for their effect upon counterion binding.