Water-soluble thermosensitive polymers were prepared by copolymerization of acryloyloxyethyl trialkyl phosphonium chlorides (AETRs) with having varying alkyl lengths, N-isopropylacrylamide (NIPAAm), and without/with butyl methacrylate (BMA). The relative viscosity of the solutions of the AETR-NIPAAm copolymers increased with increasing AETR content. The thermosensitivity of the AETR-NIPAAm copolymers was affected by an introduction of hydrophobic BMA into the copolymers and the thermosensitivity of the AETR-BMA-NIPAAm copolymers increased with chain length of alkyl groups in phosphonium groups in AETR. The AETR-NIPAAm copolymers and the AETR-BMA-NIPAAm copolymers had high flocculating ability against kaolin or bacteria such as Escherichia coli. The flocculating ability of the copolymers was affected with the AETR content, BMA content, and the chain length of alkyl groups in phosphonium groups. Almost 100% of the AETB-BMA-NIPAAm and AETO-BMA-NIPAAm copolymers dissolved in water could be recovered by heating the solution above 30degreesC in the presence of NaCl. (C) 2003 Wiley Periodicals, Inc.