Polyurethane/polyacrylonitrile semi-interpenetrating polymer network containing varying weight percent of guar gum were prepared. The resulting composites were subjected to biodegradation with the help of specific fungi Aspergillus niger. The composite before and after biodegradation were subjected to wide angle X-ray scattering studies. The nanocrystallite regions, area, and size are determined from X-ray data using three different asymmetric column length functions. A comparison of these parameters does explain the nature of biodegradation at microlevel. (C) 2008 Wiley Periodicals, Inc.