Solid-liquid phase equilibrium and solubility data for quaternary system of terephthalic acid + isophthalic acid + phthalic acid + N-methyl-2-pyrrolidone at 303.15 K and 313.15 K were determined by Schreinemakers' method of wet residue under atmosphere pressure. Based on the measured solubility, the quaternary phase diagrams were constructed at the two studied temperatures according to the Janeck method. At each temperature, the quaternary phase diagram includes three crystallization regions of pure solid, three co-saturated curves and one eutectic point. The three pure solids are phthalic acid, adduct of isophthalic acid with N-methyl-2-pyrrolidone (the mole ratio of isophthalic acid to N-methyl-2-pyrrolidone is 1:2), adduct of terephthalic acid with N-methyl-2-pyrrolidone (the mole ratio of terephthalic acid to N-methyl-2-pyrrolidone is 1:2), which are confirmed by the method of Schreinemakers' wet residue. The crystallization region of adduct of terephthalic acid with N-methy12-pyrrolidone is larger than those of phthalic acid and adduct of isophthalic acid with N-methyl-2pyrrolidone. Furthermore, the quaternary solid-liquid phase equilibrium were predicted by NRTL model. The calculated quaternary phase diagrams agreed well with experimental data. (C) 2015 Elsevier B.V. All rights reserved.