For calculating the interfacial tension of ternary polyethylene glycol (PEG)-dextran (DEX) and PEG-salt aqueous two-phase systems (ATPS), an extended interface phase model is developed. In the model, the concept of interface phase is combined with the modified NRTL model proposed in a previous paper (Wu et at., 1998), and the effect of electrical potential on the interfacial tension for electrolyte solutions is also taken into account. The model needs only one adjustable parameter, beta, for representing simultaneously the interfacial tension of several ATPS which are in the same series and differ only in the polymer molecular weights. It is found that the model can describe not only some experimental results qualitatively, such as the dependence of interfacial tension on the tie-line length, the concentration difference of components between two bulk phases, and the polymer molecular weights, but also can be at least used to predict the experimental interfacial tension semi-quantitatively, although the values are very small in magnitude (10(-4)-1.0 mN m(-1)).