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The solid−liquid phase equilibrium and mutual solubility for the ternary 3-chlorophthalic acid 4-chlorophthalic acid water system were determined at (283.15 and 313.15) K. The phase diagrams of the system were constructed on the basis of the measured solubility. The solid phases formed in the studied system were confirmed by Schreinemaker’s method of wet residues. At (283.15 and 313.15) K, both 3-chlorophthalic acid and 4-chlorophthalic acid were formed in the ternary 3-chlorophthalic acid 4-chlorophthalic acid water system. Besides, an adductive compound with the formula (3-C6H3Cl(COOH)2·4-C6H3Cl(COOH)2) existed. The crystallization field of 3-chlorophthalic acid was larger than that of the adduct or 4-chlorophthalic acid at each studied temperature. The solubility data and the ternary phase diagram for the system of 3-chlorophthalic acid 4-chlorophthalic acid water at (283.15 and 313.15) K can provide the fundamental basis for the preparation of 3-chlorophthalic anhydride from 3-chlorophthalic anhydride and 4-chlorophthalic anhydride mixtures. [ABSTRACT FROM AUTHOR]