SYNOPSISPolyacrylonitrile ( PAN) fiber was analytically characterized to identify the structural changes that occur due to atmospheric degradation. Degradation structures comprised of C=C, C=N, and C=O species were identified using infrared spectroscopy. A significant decrease in the molecular weight was measured using size exclusion chromatography. PANacrylate copolymer fibers were analyzed and found to exhibit the same type of structures and changes in properties upon atmospheric degradation.