Abstract: The effect of alcohol on the aqueous two-phase system properties of SDS/CTAB/H₂O/Na₂SO₄ system was studied by the pseudo ternary phase diagram. The electrostatic forces (both attraction and repulsion) between cationic surfactant and anionic surfactant decreased because of the interaction of alcohol hydroxyl group and surfactant polar group. As a result, the phase area width of aqueous two-phase system increased. Long chain alcohol (more than five carbonatoms), which was in high concentration in the SDS/CTAB/H₂O/Na₂SO₄ system, caused the anionic aqueous two-phase system area to vanish and the cationic aqueous two-phase system area to become wider. The CTAB rich phase turned from top phase to bottom phase.