Abstract: Samples of glass fiber S2-epoxy (JF-43)prepreg were exposed to 45℃ for different periods up to 56 days.They were analyzed in depth using various techniques to determine the extent of the chemical changes occuring.The physicochemical techniques used were infrared spectroscopy(IR),thermal analysis (differential scanning calorimetry and chermogravimetric analysis),steric exclusion chromatography.All showed evidence of significant changes,the most sensitive one being the infrared spectroscopy.IR test showed that the number of unreacted epoxy groups decreases steadily at a rate of 6.3% per day during the initial 14 days,which is similar to volatile content of resin (4.4%) and insoluble matter content of resin(4.6).The heat of polymerization of resin,measured by thermal analysis,decreased by 14.3% per day from its initial value during the initial 14 days.