TG-DTA-EGA simultaneous measurements were made for polyimide isothermally at 498 and 541 degrees C in simulated air, in order to confirm the previously found facts by TG-DTA that below 520 degrees C the mass loss is a single step process but above 550 degrees C it proceeds by a two step process. All detected volatile products are carbon dioxide, carbon monoxide, water, nitrogen and nitrogen monoxide at the both temperatures, but the volatilization behavior at 541 degrees C is different from that at 498 degrees C, as was expected from the previous results. Thus the previous postulate was confirmed. However, because volatilization of water at 498 degrees C differs from that of nitrogen monoxide, the process in the lower temperature range really proceeds by a two step process. On the other hand, at 541 degrees C the volatilization rates of all products change similarly to each other and also to the mass loss curve and the DTA curve.