The reactions of octenyl succinic anhydride (OSA) with three physical forms of starch-insoluble granular waxy maize starch, granular microporous starch, and soluble maltodextrin-were investigated and compared. Effects of OSA level (3%, 9%, 15%, and 50% based on the weight of starch) on reaction efficiency (RE) also were examined. As OSA concentration increased, degree of substitution (DS) of all OSA-modified starches increased, but RE decreased. Octenyl succinic maltodextrin had a greater DS and RE than OS granular waxy maize starch prepared at the same conditions, indicating that maltodextrin had more reaction site available. DS was the highest (0.27) for OS maltodextrin but only 0.12 for granular waxy maize starch. Increasing the surface area of starch granules by creating pinholes increased RE at OSA concentrations of 9%, 15%, and 50%. Extensive 13 C and 1 H nuclear magnetic resonance analysis indicated that structures of OSA-modified granular starch and maltodextrin were different. In granular starch, most OS substitution occurred at the O-2 and O-3 positions; in the maltodextrin, O-2, O-3, and O-6 positions were substituted, even at the reducing end.