Solubilities of isophthalic acid in ternary mixtures of acetic acid + water + benzoic acid were determined by the static analytical method. The temperature range was from 292.55 to 372.10 K, the mass ratios of water to acetic acid were 0, 1:9, and 1:4, and the mass fractions of benzoic acid in initial solvent ranged from 0 to 20.1%. The experimental results indicated that the solubility of isophthalic acid increased with the introduction of benzoic acid to the binary solvents of acetic acid and water, and that could be explained by Hansen's three-dimensional parameters and like-dissolves-like rule. Then the experimental solubilities were correlated by the nonrandom two liquids (NRTL) model, and the model parameters were regressed. The values calculated by the NRTL model showed good agreement with the experiment data.