The pervaporation separation properties for the MTBE/C-5/methanol ternary system and three corresponding binary mixtures were measured with a cellulose acetate membrane. It was found that there are very high pervaporation properties for methanol/C-5 (the flux of methanol >430 g/m(2).h). However, the flux of methanol for methanol/MTBE is lower than go g/m(2).h. The methanol concentrations in the permeate are similar for the two binary mixtures. From the result of ternary mixtures, the flux of methanol decreased from about 450 to 100 g/m(2).h, with an increase of MTBE concentration in the feed from 5 to 50 wt%, and there is a strong accompany effect between methanol and MTBE. Based on Fick's law and the accompany effect, the models of pervaporation flux and quasi-phase equilibrium of permeate components for the ternary mixture were advanced. The calculated values agreed with the experiment results. The models will be useful for separation process design of the MTBE/C-5/methanol ternary mixture.