An aqueous pyridine solution was dehydrated by using modified polyacrylonitrile membranes to form in situ complexes with the solute in the feed solution, The pervaporation separation of a water-pyridine mixture using poly(acrylonitrile-co-acrylic acid) membranes exhibited a separation factor ranging from 13.4 to 2207 and a flux of 45-610 g/m(2) h depending on the operating temperature and the concentration of the feed mixture. The permeation mechanism through the membrane is explained on the basis of the molecular interaction between the permeants and the membrane.