Different from a single-surfactant system, a three-phase body consisting of excess water, sur (microemulsion), and excess oil phases is largely skewed to higher temperature with decreasing total surfactant concentration in a multisurfactant system of water/R2EO2/R12EO4/R12EO6/R12EO8/heptane, where R12EO(n) indicates homogeneous polyethylene glycol dodecyl ether. The surfactants are mainly distributed between a microemulsion phase and an excess oil phase. Inside the microemulsion, the surfactants are considered to exist in the micro-oil domain and at the interface between the bicontinuous oil and water domains, if the solubility of surfactant in water is negligible. Considering this condition, the mixing ratios of surfactants at the interface were obtained using the phase diagram and the solubilities in the excess oil phase at each temperature. It is confirmed that the weight additivity of three-phase (HLB) temperatures of each surfactant hold in this mixed surfactant system. The meaning of the weight addivitiy is discussed according to the correlation between R-theory, hydrophilic-lipophile-balanced (HLB) number, and HLB temperature. It was found that there is a compensating tendency of solubilities of each surfactant in oil phase. With the rise in temperature, the solubilities of lipophilic surfactants largely decrease although that of hydrophilic surfactant increases. As a result, the total solubility of the mixed surfactant decreases at higher temperature.