A collagen was selected to study the membrane separation of the water/ethanol mixture by pervaporation. The membrane showed a preferential permeation of water, with a separation factor alpha = 50 (at 50 wt%). After being doped into a chromium sulfate aqueous solution, the membrane was tanned easily with a homogeneous chromium distribution, and the resultant showed excellent permselectivity (ca. alpha = 250) with only a slight depression of the water flux. The gradient-tanned membrane was prepared by the pervaporation technique. This membrane showed a close value (ca. alpha = 210, maximum value ca. alpha = 450) to that of the doped membrane, even by a short contact period with the tanning solution.