The adsorption of ammonium and nitrate by temperature-stimulus-responsive poly(N-isopropylacrylimide) (NIPA) gel and poly(N-isopropylacrylamide-co-chlorophyllin) (NIPA-CH) gel in different states was investigated. Both the NIPA gel and NIPA-CH gel could adsorb ammonium and nitrate in a swollen state (swollen gel) and a swelling state (swelling gel), and they adsorbed ammonium more than nitrate. When the gels were shrinking (shrinking gel), they could adsorb a little ammonium from solution, but when the gels were in a shrunken state (shrunken gel), they hardly adsorbed ammonium. The adsorption of both ammonium and nitrate increased for the swelling NIPA gel in comparison with the swollen gel. The NIPA-CH gel was the opposite in this respect. The difference in the amounts of adsorption of ammonium and nitrate by the swollen and swelling NIPA-CH gels was more significant than that of the NIPA gels. It was suggested that ions such as ammonium and nitrate could not diffuse into the gels freely. The adsorption of ammonium and nitrate was affected not only by the phase transitions of the gels but also by the electrical charges. The experimental results for the adsorption of ammonium and nitrate during the volume changes of the gels imply that if the gels are applied to the immobilization of microorganisms, they may improve mass transfer between the immobilization matrix and bulk liquid tinder cyclic temperature changes and promote reactions of the immobilized microorganisms, especially the nitrification of nitrifying bacteria immobilized by the NIPA-CH gel. (c) 2005 Wiley Periodicals, Inc.