Thermosensitive copolymers were prepared by graft copolymerization of N-isopropylacrylamide onto poly(vinyl alcohol) in dimethyl sulfoxide (DMSO) using potassium peroxo-disulfate as an initiator. The phase transition temperature was measured by differential scanning calorimetry. The copolymers exhibited almost the same transition temperature (about 33-degrees-C) as that of poly(N-isopropylacrylamide) regardless of the composition of the copolymers. The copolymer membranes were obtained by evaporating solvent from the DMSO solution of the graft copolymers and were insolubilized by annealing the membranes at 120-degrees-C for 10 h. Permeation of the lithium ion and Methylene Blue through the membranes was investigated at various temperatures. The permeation of solutes was greatly affected by temperature, i.e., the permeation of the solutes could be controlled at temperatures below and above 33-degrees-C.