Solubilities of three binary systems and five ternary systems involving terephthalic acid, p-toluic acid, and 4-carboxyl-benzaldehyde dissolved in pure acetic acid and aqueous acetic acid have been measured using a laser technique from 300 K to 440 K. The experimental data were compared with predicted results using the modified universal quasichemical functional group activity coefficient (UNIFAC) method. The results show that the modified UNIFAC model alone is not reliable for systems with strong interactions between molecules.