In this investigation, the solid-liquid phase equilibrium and the mutual solubility for ternary 3-nitrophthalic acid-4-nitrophthalic acid-water system were determined at 283.15 K and 333.15 K. The phase diagrams of the system were constructed based on the measured solubility. The solid phases formed in the studied system were confirmed by Schreinemakers' method of wet residues. In addition, the density of equilibrium liquid phase was obtained. At 283.15 K and 333.15 K, both 3-nitrophthalic acid and 4-nitrophthalic acid were formed in the ternary 3-nitrophthalic acid-4-nitrophthalic acid-water system. Adductive compounds existed: 3:1:1 adduct 3-C6H3NO2(COOH)(2)center dot 4-C6H3NO2(COOH)(2)center dot H2O at 283.15 K and 1:1 adduct 3-C6H3NO2(COOH)(2)center dot 4-C6H3NO2(COOH)(2) at 333.15 K. The adduct had a bigger crystallization field than 3-nitrophthalic acid or 4-nitrophthalic acid at each studied temperature. The solubility data and the ternary phase diagram for the system 3-nitrophthalic acid-4-nitrophthalic acid-water at 283.15 K and 333.15 K can provide the fundamental basis for preparation of 3-nitrophthalic acid from 3-nitrophthalic acid and 4-nitrophthalic acid mixtures. (c) 2008 Published by Elsevier B.V.