The liquid-liquid equilibria (LLE) data for the quaternary system, acetic acid, water, p-xylene, and ethyl acetate, was obtained at 313.15 K under atmospheric pressure. The composition distribution of each component on the aqueous phase and organic phase were investigated. To examine the effects of the solvent, the LLE phase diagrams at different p-xylene to ethyl acetate ratios are presented for this quaternary system. The reliability of the tie-line data of this study was also confirmed using the Othmer-Tobias and Hand equation. The distribution ratio and separation factors for the extraction of acetic acid from the aqueous solutions were obtained. Furthermore, the equilibrium data of this study was analyzed using the quasichemical activity coefficient (UNIQUAC) and nonrandom two liquid (NRTL) models. The experimental data was correlated successfully with both the activity coefficient models that were extended with each binary interaction parameter. To determine the accuracy of each activity model, the root-mean-square deviations (RMSDs) and average absolute deviation (AAD) resulting from these models were also calculated. The RMSDs and AAD of the NRTL models were 0.0196 and 0.0213, respectively, whereas those of the UNIQUAC model were 0.0166 and 0.0114, respectively. These values showed that the UNIQUAC model is superior to the NRTL model.