Cationic starch, an important additive for papermaking and other uses, was prepared from native cornstarch and 3-chloro-2-hydroxypropyltrimethylammonium chloride in aqueous sodium hydroxide using a twin-screw extruder as the reactor. The effects of physical and chemical reaction variables on reaction efficiency (RE) and degree of substitution (DS) of up to 0.05 were studied. RE was based on nitrogen content contributed by cationization of the starch after the products were exhaustively washed to remove unreacted monomer. Under certain reaction conditions, exceptionally high RE (90%+) was achieved, exceeding maximum values previously reported using laboratory-batch reaction procedures. The combined effects of high reaction temperature (90 degrees C), intense mixing, high-starch solids (65%), and appropriate levels of catalyst contributed to the unusually high RE values achieved by the reactive extrusion process.