The effects of elevated temperature aging on the microstructural changes of isotactic polypropylene matrix in a composite have been studied using wide-angle X-ray scattering (WAXS) and Fourier-transform infrared spectroscopy (FTIR). The objective was to quantify small and slow changes in crystallinity due to thermal aging. To minimize sample variability, polypropylene resin was extracted from the molded composite plaque. Changes in crystallinity level and crystalline form were detected using WAXS after prolonged aging at 90 and 140 degrees C. FTIR was utilized to monitor in-situ crystallinity changes and to detect oxidation products due to thermal decomposition. The level of crystallinity was monitored by changes in the absorbance ratio of A(997)/A(973) and A(841)/A(973); the former ratio was found to be more sensitive for detecting crystallinity changes. Aging at 140 degrees C resulted in oxidation. The kinetics of secondary crystallization for the aging conditions studied was characterized using Avrami plots.