Abstract: This study was about the cloud point of alkyl-polyoxyethylene (AEO₉) and aqueous two-phase systems formed by AEO₉ aqueous solution. It was shown that the cloud point of AEO9 aqueous solution was related to the composition and aqueous two-phase systems would be formed when the temperature was above the cloud point. Salt has an important influence on the properties of AEO₉ solution. It could lower the system cloud point and reverse surfactant-rich phase and surfactant-leau phase of aqueous two-phase system.Methylene blue could be extracted to some extent in the aqueous two-phase system of AEO₉. If the system contained ionic surfactant, the distribution coefficient (D) of methylene blue would be changed. Cationic surfactant could make D larger and anionic surfactant could make D smaller.The mechanism behind extraction in ATPS was discussed.