4-(Phenylethynyl-alpha,beta-C-13)phthalic anhydride (PEPA) and C-13-labeled phenylethynyl-terminated imide (PETI) oligomers were synthesized, and solid-state C-13 nuclear magnetic resonance (NMR) spectroscopy was used to determine the structure of cured oligomers. Solid-state C-13 NMR spectra were collected before and after thermal curing. Using solid-state C-13 NMR difference spectroscopy, several cure products were identified. The observed C-13 NMR resonances were assigned to four different classes of cure products: aromatics, products from backbone addition (substituted stilbenes and tetraphenylethanes), polyenes, and cyclobutadiene cyclodimers. The effects of postcuring and oligomer chain length on the structure of the cured resins were examined.