We studied the relationship between the mixing ratio of surfactants adsorbed at the water/oil interface (Copt) and the weight ratio of oil/(water + oil) (R-ow) inside the middle microemulsion and the difference between the C-opt and the mixing ratio of surfactants in the whole surfactant (W-opt). The three-phase plane method was employed to determine the C-opt and the monomeric solubilities of surfactant in water and oil (S-w and S-o) in water/mixed poly(ethylene glycol) alkyl ethers (CmEOn)/hydrocarbon toil) systems. A linear relationship can express the relationship between the C-opt and the R-ow over a range of 0.2 < R-ow < 0.8, while the S-w and S-o are almost unchanged. In the HLB number (N-HLB) scale, the curves of C-opt vs R-ow lie between N-HLB = 10 and II for the decane system and near N-HLB = 12 for the cyclohexane system. The W-opt deviates from the C-opt: the deviation is a function of C-opt and the secession ratio of surfactant, i.e., the ratio of(lipophilic) surfactant desorbed from the micro interface into oil to the total surfactant.