The imidization of polymerizable reactive mixtures, PMR-15 has been performed in a vacuum oven at isothermal aging temperatures ranging from 65 to 200 degrees C for aging periods of 0.5 to 2.5 h. The weight loss of the resin and chemical changes that occurred as a result of aging were monitored gravimetrically and by FT-IR spectroscopy. Differential scanning calorimetry was used to determine the temperature at which imidization took place. Imidization nas observed to commence at 65 degrees C after long aging times, t greater than or equal to 2.5 h and at similar to 95 degrees C at a shorter time, t similar to 0.5 h, At higher aging temperatures of 135 to 165 degrees C, extensive imidization occurred, This was shown by the dramatic increase in imide absorption bands at 1780 and 1380 cm(-1). Beyond 165 degrees C, there were no significant changes in the imide absorption bands, suggesting that imidization was nearly complete. The activation energy for isothermal aging mas determined from the slope of the log of the rate of weight loss vs 1/T curve to be similar to 4.5 kJ/mol and is lower than the average activation energy for imidization similar to 43 kJ/mol obtained from the plot of the log of the rate of increase of the imide carbonyl peak absorption at 1780 cm(-1) vs 1/T.