Increase in mass transport resistance is one of the major difficulties in immobilization by the use of gels. Functional gels can change their volume and inner structure depending on ambient conditions as stimuli and may reduce the difficulty as a novel immobilization matrix. The response of poly N-isopropylacrylamide-co-chlorophyllin (NIPAAm-Ch) gel to temperature changes and its performance on nitrification with nitrifiers immobilized by the gel were investigated in a continuous-flow stirred-tank reactor (CFSTR) with cyclic temperature changes. The gel with an immobilized nitrifier swelled and shrank alternately with 1.2-1.6-fold volume change under a cyclic temperature change of 32-36 degrees C with a period of 2 or 4 h in the reactor. Volume changes of the gel brought periodic changes of its structure that accelerated dissolved oxygen transfer and concentrated ammonium into, and as a result, promoted nitrification compared with that at constant temperature. (c) 2006 Wiley Periodicals, Inc.