Selective transport by pervaporation of physico-chemically similar and polar components through a polar membrane is complicated due to competitive sorption and diffusion phenomena. In the present study the mechanism of preferential sorption has been predicted for such a system, methanol-ethylene glycol-cellophane. The mechanism has been explained in terms of intercrystalline swelling of the polymer matrix in presence of increasing methanol concentration in the feed. The reduction in preferential sorption at increasing wt% of methanol in the feed may be due to the increasing accessibility of the membrane towards ethylene glycol. This phenomenon has been quantitatively explained by considering a non-linear dependence on concentration of the binary liquid-polymer interaction parameter. Theoretical sorption data have been derived from the Flory-Huggins thermodynamics by using the swelling equilibrium condition. The coupling and plasticization phenomena in sorption are explained in terms of liquid-polymer interaction parameters. The theoretical results show good agreement with previously published experimental data.