Imidization of PMR-15 was investigated using Fourier transform infrared spectroscopy (FTIR) as a function of time and temperature. Imidization was performed at 65 less than or equal to T less than or equal to 300 degrees C for 3 less than or equal to t less than or equal to 150 min. FTLR spectroscopy showed that imidization (measured by the changes in the imide carbonyl absorption at 1778 sm(-1)) increased with temperature and time. Imidization was found to be nearly completed in 2.5 h at 300 degrees C. Imidization of PMR-15 occurred in three stages : (i) the initial imidization region characterized by gradual reaction followed by (ii) a very rapid reaction region that spans about 0.5 h and (iii) a final imidization region characterized by a gradual reaction and spans about 2 h. An Avrami-type kinetic analysis was used to obtain the reaction order of 1.5 and 1.7 and the rate constant for imidization of 1.3 x 10(-3) and 1.5 x 10(-3) min(-3/2) at 135 and 165 degrees C, respectively. Comparison with other kinetic models shows agreement at low conversions (p less than or equal to 15%). At high conversions of p > 20%, a second-order kinetic model seems to fit the data reasonably well in agreement with the observed 3/2 order.