Phase diagrams of sodium perfluorodecanoate-water and sodium decylsulfonate-water systems were drawn from changes of solubility and critical micelle concentration (CMC) with temperature. The mixed CMC values of the two surfactants of different compositions were determined and two kinds of micelles were found to be formed; one is a fluorocarbon-dominant micelle and the other is a hydrocarbon-rich micelle with some fluorocarbon surfactants. The precipitate from aqueous solution containing the two surfactants below the micellization temperature of the mixed surfactants was analyzed by X-ray diffraction and was found to be a mixture of the two surfactant crystals, Analysis was made of the temperature dependence of the electrical conductivity of an aqueous solution containing suspended precipitates which were formed at low temperature from solution of a certain molar ratio of the two surfactants, and the phase diagram of the three-component system was constructed. The three-dimentional phase diagram was quite different from that of mixed surfactants which are able to form mixed micelles of any composition.