Compaction of TiO2 suspended particles utilizing the hydrophilic/hydrophobic transition of a cationic thermosensitive copolymer of N-isopropylacrylamide (NIPAM) and N,N-dimethylaminopropylacrylamide (DMAPAA) was examined. The surface of suspended TiO2 particles were sufficiently covered with the adsorbed polymer molecules at room temperature, and the suspension was subsequently heated above the transition temperature of the polymer. The thrusting motion of a plunger was used in order to provide an adequate mechanical force to compact the TiO2 suspended particles. The transition temperature was dependent on the DMAPAA content in the copolymer and the pH of the suspension. The transition temperature increased with increasing DMAPAA content and decreased with increasing pH. While suspended TiO2 particles were not compacted with poly(NIPAM), hard compacted sludge was readily obtained for cationic copolymer even if the content of DMAPAA groups was as low as 0.23 mol%. Furthermore, residual polymer molecules in the supernatant were not detected even if the polymer dosage was increased. These findings were attributed to the hydrophobic interaction of thermosensitive copolymer molecules adsorbed on TiO2 particles. (C) 2004 Elsevier Inc. All rights reserved.