Solid-liquid phase equilibria (SLE) data are essential in designing apparatus and operation of industrial crystallization because they determine the possibility of separation and predict purification limits. In this study, a SLE determination system by using differential scanning calorimetry (DSC) was improved and applied not only to eutectic systems but also to partial solid solution systems. This method is different from conventional techniques that use onset and peak temperatures to determine SLE. A classification method of phase diagram types is proposed. This determination system was applied to the naphthalene (naph)-p-chloronitrobenzene (p-CNB) and naph-catechol (cate) systems (eutectic systems) and the indole-naph system (partial solid solution system). From the calculated results, the determined SLE was in good agreement with the literature data.