AbstractThe three‐phase behavior in the quaternary system ofn‐dodecyl polyglucoside C12G1.46/1‐butanol/cyclohexane/water has been studied at 40°C in terms of the variables γ and δ. Increasing δ at constant γ causes a phase inversion from an oil‐in‐water microemulsion in contact with excess oil (winsor I or 2) to a water‐in‐oil microemulsion in contact with excess water (winsor II or 2−) via a middle‐phase microemulsion in contact with excess oil and water (winsor III or 3). By taking into account the different solubilities of alkyl polyglucoside and 1‐butanol in the oil phase, the composition of the hydrophile‐lipophile balanced interfacial film in the middle of the three‐phase body can be calculated. The effects of different oils and aqueous media on the phase behavior and on the composition of the interfacial film and the efficiency for alkyl polyglucoside to make equal weights of water and oil to a single phase were investigated. It was found that the oil molecules with small molecular volumes can improve the solubilizing efficiency of the surfactant to form single‐phase microemulsion. In inorganic salt (NaCl) and add (HCl) solutions, less 1‐butanol is needed than that in alkali (NaOH) solution to