The thermosensitive polyvinyl alcohol-graft-N-isopropylacrylamidemethacrylic acid (PVA-g-NIPAAm-MAc) terpolymer membranes containing carboxyl groups were prepared. The swelling ratios of the membranes were measured at various temperatures. The temperature dependence of the swelling ratios of the terpolymer membranes was different from that of PVA-g-NIPAAm copolymer membranes. The swelling ratios of PVA-g-NIPAAm-MAc (5-15) (wt % in feed) increased with increasing temperature up to 35-38 degrees C, then decreased. However, the swelling ratio of PVA-g-NIPAAm-MAc (30-50) terpolymer membranes did not depend on temperature in the temperature range of 10-48 degrees C. To clarify the swelling behavior of the PVA-g-NIPAAm-MAc terpolymer membranes, the swelling ratios of the PVA-g-NIPAAm-Acrylic acid (AAc) terpolymer membranes, the viscosity, and optical density of various polymer solutions were measured. The different swelling behavior of PVA-g-NIPAAm-MAc (or AAc) terpolymer membranes from that of PVA-NIPAAm copolymer membranes was thought to be due to hydrogen bonding between amide groups in NIPAAm moieties and carboxyl groups in MAc (or AAc) moieties in the terpolymer membranes and the difference of swelling behavior between PVA-g-NIPAAm-MAc and PVA-g-NIPAAm-AAc terpolymer membranes was thought to be brought about by hydrophobic interaction due to methyl groups in PVA-g-NIPAAm-MAc terpolymer membranes.