Experiments were performed on pervaporation of methanol-MTBE mixtures through cellulose acetate (CA) and triacetate (CTA) membranes. The total flux for CA was lower than for CTA, whereas the selectivity for CA was higher than for CTA. For CA, the permeation of methanol seems to be unaffected by MTBE, whereas the permeation of MTBE is apparently affected by methanol through the plasticizing effect. For CTA however, both methanol and MTBE show complex coupling phenomena. These phenomena are successfully explained by the Flory-Huggins theory and the solution-diffusion model.