The mutual solubility for ternary systems of terephthalic acid + isophthalic acid + N-methyl-2-pyrrolidone, phthalic acid + isophthalic acid + N-methyl-2-pyrrolidone, and phthalic acid + terephthalic acid + N-methyl-2-pyrrolidone were measured at 303.15 K and 313.15 K under normal pressure. Six phase diagrams were plotted on the basis of the measured solubility data. Two pure solids were formed for each system, and they were recognized by the method of Schreinemakers' wet residue. Two crystalline adducts were formed, which were the adduct of isophthalic acid with N-methyl-2-pyrrolidone (named as N, isophthalic acid/N-methyl-2-pyrrolidone = 1:2 in mole ratio) and adduct of terephthalic acid with N-methyl-2-pyrrolidone (named as M, terephthalic acid/N-methyl-2-pyrrolidone = 1:2 in mole ratio). Each phase diagram exhibited only one eutectic point. The crystallization fields of phthalic acid, adduct N and adduct M were increased as the temperature increased. At the same temperature, the crystallization field of adduct M is the largest. In addition, the densities of the equilibrium liquid phase were obtained.