Chitosan films were acylated under heterogeneous conditions in methanol with acetic and hexanoic anhydrides and characterized by proton nuclear magnetic resonance, elemental analysis, and multiple internal reflective Fourier transform infrared spectroscopy. The disappearance of the NH2 vibrational band at 1590 cm(-1), the appearance of the amide II band at 1555 cm(-1), and the relatively low intensity of the ester band at 1735 cm(-1) showed that acylation at the surface was site-selective for the amino (N) functionalities. Furthermore, N-acylation at the surface region appeared complete within 1 h. The acylated chitosan films were fractionated in aqueous acetic acid for compositional analysis. Acetylation of chitosan films for 3 h gave 52% of aqueous acetic acid insoluble chitin (outer film region) and 48% unreacted chitosan. In contrast, 3 h hexanoylation reactions resulted in <1% hexanoylation and a >99% aqueous acetic acid soluble product. Thus, film N-acetylation was more rapid than N-hexanoylation. Moreover, acetylation resulted in the formation of discrete outer chitin layers and an unreacted chitosan interior. The thickness of these film regions with different compositions may be controlled by the reaction conditions. Biodegradation studies of the acylated chitosan films carried out in laboratory-scale aerobic thermophilic compost reactors revealed that the formation of chitin at the film surface enhanced the biodegradability of the films. Specifically, the 3 h acetylated chitosan film (0.045 mm thickness) showed 100% weight loss during a 28 day exposure, whereas unmodified chitosan showed no significant weight loss after 35 days.