In this article, we report the surface modification of branched polyethyleneimine (PEI) for improved biocompatibility. PEIs with different surface functionalities were synthesized via covalent modification of the PEI amines, including neutralized PEI modified with acetic anhydride, negatively charged PEI modified with succinic anhydride, hydroxylated PEI modified with glycidol, and PEIpoly(ethylene glycol) (PEG) conjugates modified with both PEG and acetic anhydride. The modified PEI derivatives were characterized with 1H-NMR, Fourier transform infrared spectroscopy, and -potential measurements. An in vitro cytotoxicity assay of mouse fibroblasts revealed that the biocompatibility of PEI was significantly improved after these modifications. The neutral and negatively charged PEIs were nontoxic at concentrations up to 200 g/mL, whereas the pristine PEI was toxic to cells at concentrations as low as 10 g/mL. The successfully modified PEIs with different surface charges and functionalities may provide a range of opportunities for various biomedical applications. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013