An oxidoreduction-sensitive polymer, poly[3-carbamoyl-1-(p-vinylbenzyl)pyridinum chloride] (PCVPC), was grafted on a porous membrane, and the water permeation through the membrane was investigated. PCVPC homopolymer was found to be water soluble in the oxidized state but insoluble in the reduced state. It could be reversibly dissolved and precipitated in aqueous solutions of high ionic strengths. The polymer was grafted onto a glow-discharged fluoropolymer membrane. In the oxidized state the grafted chains were ionized and extended to cover the pores and decrease water permeation. The rate of permeation depended significantly upon the ionic strength of the aqueous solution. In the reduced state, however, the polymer brushes were deionized and increased water permeation. These changes in permeability were reversible by oxidoreduction, when the formation of an insoluble polymer complex in the reduction process was avoided under high ionic strength. The response of polymer chains grafted on the membrane corresponded to that of polymers in solution.