A high level of control over grafting reactions onto cellulose derivatives was achieved by an acyl transfer reaction mechanism. Ethylene/maleic anhydride copolymer (EMA) and styrene/maleic anhydride copolymer (SMA) were grafted onto cellulose acetate (CA) and methyl cellulose (MC) in a simple and effective way under homogeneous reaction conditions. While pyridine, a commonly used catalyst for the reaction of an anhydride with an alcohol to form an ester, gave an inconveniently long reaction time, N-methylimidazole (NMID) and 4-(N,N-dimethylamino)pyridine (DMAP) were found to increase greatly the rate of esterification.