Dually stimuli-responsive hyperbranched polyethylenimine derivatives (HPEI-star-PPOs) were successfully synthesized through Michael addition of commercial HPEI, poly(propylene oxide) dimethacrylate, and 2-mercaptoethanol. In aqueous solution, these HPEI-star-PPOs exhibited response to temperature and pH. The corresponding lower critical solution temperature (LCST) could be readily adjusted by changing the feed ratio of HPEI to PPO, which also presents the ratio of hydrophilicity to hydrophobicity, indicating that this LCST transition is based on hydrophilic-hydrophobic balance. Because of the tertiary amine as well as unreacted primary or secondary amine that can be protonated during pH decreases, HPEI-star-PPOs exhibited a pH-dependent thermoresponse. The 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay against COS-7 cells demonstrated that HPEI-star-PPO had relatively low cytotoxicity compared to HPEI. All these characteristic suggest that this stimuli-responsive polymer is a promising functional material for biomedical applications. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 127: 3249-3255, 2013