The modified polyacrylonitrile (PAN) membranes used in this study had sulfonic acid moiety to act as a specific functional group for the dehydration of pyridine aqueous solution. The in situ complex formed between pyridine in the feed and sulfonic acid moiety in the membrane could enhance the dehydration capacity of the modified PAN membranes. It was found that the pervaporation patterns were closely related to the content of the sulfonic acid in the modified PAN membranes. The effective in situ complex membrane can be prepared by controlling the content of sulfonic acid in the membrane. The in situ complex membrane shows an increase of permeation flux with a slight decrease in separation factor as operating temperature increase.