The methods for multiphase stability determination are generally based on Gibbs free energy minimization. However, a novel and efficient method is proposed in this work for one-, two-, and particularly three-liquid-phase stability identification in ternary mixtures. The method is designated the extended general line-length (EGL) method, which is based on our previous GL method. Both the EGL and GL methods are equation-solving approaches. The EGL method is easy to understand and can be performed fast. Only parameters of the thermodynamic models, such as UNIQUAC and NRTL, are required for the stability determination. The thermodynamic model applied does not influence the rapidness and validity of the EGL method. Graphical techniques and simultaneous computation are adopted in this method. Its applicability and efficiency are exemplified by three typical examples. For each example, three different phase configurations, consisting of one-, two-, and up to a three-liquid phase, are tested. The results show that the EGL method can be used for the determination of multiphase stability in all liquid-phase configurations of ternary systems. (C) 2004 American Institute of Chemical Engineers.