Author reported a method in the previous paper on the determination of the critical micelle concentration (cmc) of a surfactant, in which a strongly ultraviolet absorbing organic substance (I), such as diethylaniline, was added to the aqueous surfactant solution and the cmc was obtained as the breaking (minimum) point on the curve of the λmax in ultraviolet region vs. surfactant concentration.In the present work, this method was applied to more complex cases, and good results were obtained.When a surfactant had little or no ultraviolet absorption characteristics, such as tetradecyldime-thylbenzylammonium chloride or sodium dodecyl sulfate, the λmax vs. surfactant concentration curve similar to that given in the previous report was obtained even when it contained a builder or impurities, and the concentration at the breaking point agreed well with the cmc determined by other methods. A similar curve was also obtained and the cmc was clearly determined in the case of a binary or ternary surfactant system among these surfactants.Although it is often difficult to determine accurately the cmc of impure or commercial surfactant, it is possible with this method. For example, the cmc of somewhat purified Tween 20 was determined to be 0.117g/100ml at 22°C.When a surfactant had strong ultraviolet absorption characteristics, a slight breaking point appeared on the curve of λmax vs. surfactant concentration, and although the determination of cmc was possible, the types of surfactants to which the method is applicable were limited to pyridinium salts, polyoxyethylene nonyiphenyl ethers etc. (II). The determination was also possible by the addition of (I) to (II), but it was unsuitable when λmax of (I) and (II) overlapped. For example, acetophenone was suitable as (I) for the determination of alkylpyridinium salts. In regard to the determination of alkylphyridinium salt, two breaking points appeared on the curve and the concentration of the lower point agreed with the cmc obtained by another method in the case of both single system and mixed system with other substances.This method is based on the solvent effect for (I) or ultraviolet absorbing portion of the surfactant molecule, and from the results of above-mentioned measurements, it was assumed that the pyridin nucleus of the molecule of alkylpyridinium salt forming the micelle was partially contracted to the water phase, and the micellar state changed considerably with the change of the salt concentration above the cmc.