The thermally induced cross-linking of nadic-end-capped polyimides has been studied by a variety of spectroscopic techniques. Two nadic-end-capped polyimides were subjected to 371 degrees C heat treatment for 100 h in order to study cross-linking resulting from elevated temperature. The glass transition temperature of the treated samples increased to greater than 400 degrees C, indicating that both polyimides underwent extensive chemical changes during heat treatment. C-13 spin counts were measured before and after heat treatment by C-13 cross-polarization magic angle spinning nuclear magnetic resonance. Electron spin resonance measurements showed an increase in the concentration of stable free radicals in the treated samples. Dynamic nuclear polarization and electron spin resonance results show that two species of free radicals are present in the treated samples; the first is located on the polymer backbone and the second on the cross-linking network that arises from the reaction of the nadic end-capping groups. On the basis of these results, new cross-linking structures of nadic-end-capped polyimides before and after heat treatment at 371 degrees C are proposed.