Thermoresponsive polyphosphoesters bearing acetoxymethyl (AM) groups were newly synthesized. The polymers had the LCST-type cloud point in aqueous media, and the thermoresponsive phenomena was influenced by the condition of the medium. About 40% of the AM groups in the polymers were cleaved in contact with esterase for 24 h in phosphate buffer solution. This was significant compared with the spontaneous cleavage in the buffer solution. After the cleavage of the AM groups, the thermoresponsivity of the polyphosphoesters was diminished.