The effect of molecular weight of oil on the three-phase behavior and maximum solubilization was investigated in microemulsion systems of mixtures of di- and octaethylene glycol dodecyl ethers and mixtures of hexanol and octaethylene glycol dodecyl ether at constant temperature (25 or 35 degrees C). It was found that the minimum weight fraction of surfactant to make equal weights of water and oil to a single phase (X(b)) increases with increasing molecular weight of oil (hydrocarbon). Nonionic surfactant mainly distributes between micro-oil domains and the interface between microwater and oil domains inside the microemulsion phase. Surfactant molecules at the interface are responsible for the solubilization. Assuming that the monomeric solubility of each surfactant in oil is the same as that in the micro-oil domains of the microemulsion and the microwater domains consist of pure water, the weight fractions of each surfactant at the interface in the microemulsion (C-1 or C-2) were obtained by a mass-balanced equation. The mixing fraction of surfactant at the interface was also determined by another method based on the geometrical relation of the three-phase tie triangle in the composition tetrahedron. In the hexanol systems, the monomeric solubility of hexanol in water should be taken into account. C-1 + C-2 indicates the net solubilizing power and it dramatically decreases with increasing the molecular weight of oil.