醋酸與水為似共沸的混合物，因此使用傳統蒸餾無法得到高純度的醋酸。醋酸脫水，在化工程序上一般都採用共沸蒸餾之程序。分離製程中通常以醋酸酯為共沸劑(entrainer)，而本研究構想一個新的製程，即使用對二甲苯來替代醋酸酯，若此製程可行，則整個醋酸生產之成本可望大幅降低。 本研究討論水-醋酸-乙酸甲酯-對二甲苯在101.32 kPa下的相行為包括汽液相平衡及汽液液平衡。實驗部分使用改良自Othmer汽液平衡裝置進行數據的量測，在汽相之計算使用Hayden和O’Connell所提的理論考慮醋酸分子結合效應。 實驗所得來之數據使用UNIQUAC和NRTL兩種液相模式進行回歸，在汽液平衡部分此兩液相模式迴歸結果相差不多，但以NRTL較佳；並使用NRTL模式所關聯的結果來繪出此四成份系統的相圖，汽液液平衡部分則使用UNIQUAC液相模式關聯之值來畫出相圖，發現實驗點皆落於計算平面上，最後並分析四成份混合物形成汽液平衡或汽液液平衡的狀況。 Acetic acid and water is a mixture with close oiling temperature. Therefore, a conventional distillation can not produce high purity acetic acid. In chemical industry, the azeotropic distillation is applied to the dehydration of acetic acid with an entrainer. The final aim of this study is to develop a new process that replaces the old entrainer with p-xylene. In this work, the vapor liquid equilibrium (VLE) and vapor liquid liquid equilibrium (VLLE) data of water-acetic acid-methyl acetate-para xylene (p-xylene) were conducted in a modified Othmer type equilibrium still at 101.32 kPa. The measured VLE and VLLE data were correlated by the activity coefficient models of UNIQUAC and NRTL with the consideration of the association by Hayden and O’Connell in the vapor phase. Our calculated results show that the correlation by the NRTL model was in good agreement with the experimental data for VLE. The VLLE data was suitably correlated by the UNIQUAC model, but it could not be correlated by NRTL model. The diagrams showing the experimental data and the calculated composition surfaces were plotted as a function of temperature. It can be seen that the experimental points lie on the calculated surface. This indicates that the calculated and the experimental data agreed with each other fairly. The diagram of liquid composition and vapor composition excluding methyl acetate show the relation of VLE for single liquid phase or VLLE for two liquid phase in this work.