This work was to construct isothermal binary and ternary solid-liquid phase diagrams based on the measured mutual solubility data of mixtures of 2-nitro-p-phenylenediamine + ethyl acetate + isopropanol and 2-nitro-p-phenylenediamine + 4-nitro-o-phenylenediamine + N-methyl-2-pyrrolidone systems. The solubility obtained for 2-nitro-p-phenylenediamine in mixtures of (ethyl acetate + isopropanol) increased with the increase in composition of ethyl acetate in mixed solvents and temperature studied. In ternary phase diagram of 2-nitro-p-phenylenediamine + 4-nitro-o-phenyl-enediamine + N-methyl-2-pyrrolidone, it is mainly composed of two boundary curves, three crystalline regions, and one cosaturated point. The solids formed in ternary phase diagrams were confirmed via the Schreinemakers' wet residue method. The crystallization region of 4-nitro-o-phenylenediamine was smaller than that of 2-nitro-p-phenylenediamine at each temperature. The obtained solubility data in (ethyl acetate + isopropanol) were correlated with Jouyban-Acree model. Meanwhile, two activity coefficient equations (NRTL model and Wilson model) were applied to calculate the results obtained of 4-nitro-o-phenylenediamine in N-methyl-2-pyrrolidone and the mutual solubility for the ternary 2-nitro-p-phenylenediamine + 4-nitro-o-phenylenediamine + N-methyl-2-pyrrolidone system. In binary and ternary systems, the values of relative average deviation and root-mean-square deviation were no more than 4.67.00 X 10(-2) and 7.10 X 10(-3), respectively. The calculated results via these models were all acceptable for the binary and ternary solid-liquid phase equilibrium. Moreover, the application of ternary phase diagram was discussed for separation of these mixed products of structural isomerism.