The solubility of p-toluic acid (PTA) in acetic acid (HOAc) + water solvent mixtures was measured by a static method. The experimental temperature ranged from 303.2 to 363.2 K, and the mole fraction of acetic acid in the solvent mixtures ranged from 0.546 to 1.000. Together with the available solid liquid equilibrium (SLE) data for ternary systems of terephthalic acid (TPA) + HOAc + H2O and benzoic acid (BA) + HOAc + H2O, the interaction parameters of the new UNIFAC group ArCOOH, defined as an aromatic group (Ar) plus an (aromatic) carboxyl group (COOH), with an aliphatic carboxyl group (COOH), methyl group (CH3 and CH2), aromatic group (Ar), aromatic methyl group (ArCH3), and water are determined. By using the new defined group ArCOOH, the UNIFAC model predicted SLE data for the above three ternary systems agree with the experimentally determined results satisfactorily.