A series of chitosans with different molecular weights and degrees of acetylation (DAs) are reacted with nitric oxide (NO) to form [NONO](-) groups. The effects of molecular weight and DA on NO release are investigated by Griess assay. Heterogeneous reaction of NH2 groups of chitosan with NO was shown to be influenced greatly by the crystalline form of chitosan. Total NO release exhibited a bell-shaped distribution at different DAs ranging from 6 to 95%, peaking at about 50%. When DA is held constant, total NO release is directly proportional to the molecular weight. X-ray diffraction patterns indicate that the total NO release of chitosan-NO adducts is in general agreement with the intensity of reflections at low Bragg angles (2 theta = 8.6 degrees -11.1 degrees), and in turn, a relaxed hydration crystalline form and NO molecules could penetrate this crystal and react with the chitosan molecules. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 2183-2188, 2010