The states of water absorbed in the crosslinked poly(vinyl alcohol) (PVA) membranes was studied thermally using differential scanning calorimetry (DSC). It was observed that water absorbed in the membranes was distinguished into free water, freezing bound water and non-freezing bound water. As the crosslinking time increased, the nonfreezing bound water, the freezing bound water and the free water contents decreased. The bound water acted as an effective plasticizer for the polymer due to its interaction with the polymer. In the pervaporation of acetic acid-water mixture using the crosslinked PVA membranes, the decrease in the acetic acid and water permeation with increasing crosslinking time was correlated with the crosslinked structure of the membranes and the decrease in the bound water content in the polymer which led to a decrease in the plasticization effect. As a result, the separation factor increased with increasing crosslinking time.