4-Vinylpyridine was copolymerized with acrylonitrile by emulsifier-free polymerization in order to prepare a membrane for the dehydration of a water-acetic acid mixture by pervaporation. Pervaporation separation of a water-acetic acid mixture using a poly (4-vinylpyridine-co-acrylonitrile) membrane containing 1.7 mol% pyridine groups showed a separation factor ranging from 7 to 496 and a flux of 27 to 87 g/m2-hr depending on the operating temperature and the composition of the feed mixture. The permeation mechanism through the membrane is discussed on the basis of the molecular interaction between the permeants and the membrane. A catalytic transport mechanism through the membrane is proposed and discussed in detail.