By using a specially contrived experimental technique, the partition equilibrium of 4-carboxybenzaldehyde (4-CBA) between crude terephthalic acid crystals and the corresponding aqueous acetic acid solution was achieved. The equilibrium mole fractions of 4-CBA in both the liquid and solid phases were measured over a temperature range from (423.2 to 453.0) K, and the mass fractions of acetic acid in the solvent mixtures ranged from 0.800 to 0.995. The partition coefficients, defined as the ratio of the equilibrium mole fraction of 4-CBA in the solid phase to the equilibrium mole fraction of 4-CBA in the liquid phase, were experimentally determined. The experimental results show that: (1) within the temperature range of the measurements, the partition coefficient shows a decreasing trend as the temperature increases at a constant solvent composition, and (2) within the solvent composition range of the measurements, the partition coefficient increases with increasing water content in the solvents at a constant temperature. The experimental data are correlated by the nonrandom two liquids (NRTL) activity coefficient model, and the model parameters are determined in a nonlinear optimization. The fit of the correlated results is fairly good. The comparison of the predicted and experimental partition coefficients at temperatures of (423.2 to 453.0) K and the mass fraction of acetic acid in the solvent mixture at 0.950 shows that the NRTL activity coefficient model could be used as a suitable toot in predicting the partition coefficient of 4-CBA between crude terephthalic acid crystals and the corresponding aqueous acetic acid solution.