In this research, mass transport in pervaporation (PV) process of isomeric xylenes (para_xylene and ortho_xylene) was investigated by means of the resistances-in-series model. In order to obtain necessary data for the analysis, some experimental studies on separation of the xylene mixtures using a poly (vinyl alcohol) (PVA) membrane were performed. Influence of operating conditions such as para_xylene concentration in the feed and permeate pressure on the membrane performance and also the transport resistances in different layers of membrane was examined. Finally, the impact of CBr4 (Carbon tetrabromide) on the membrane performance was evaluated. The results implied that by adding CBr4 to the feed, more ortho_xylene in the permeate can be collected, because CBr4 combines with para_xylene molecules, thus ortho_xylene molecules preferably pass through the membrane rather than para-xylene molecules. The mathematical model was found to be very accurate and could be employed for prediction of PV of isomeric xylenes.