An investigation of the thermal cure reactions and the hydrolysis reactions involved in the degradation of polyimide films under temperature and humidity stress using nitrogen-15 solids nuclear magnetic resonance (NMR) is herein reported. Nitrogen-16 labeling was used in combination with dipolar decoupled, cross polarization magic angle spinning (CPMAS) NMR techniques as a means of monitoring chemical reactions as these occur in solid state polyimide. The relative concentration of each nitrogen-containing functional group was calculated using standard NMR methods based on determination of the values of the cross polarization time constant, T-HN, the proton rotating frame time constant, T-1 rho H, and observed spectral line intensities. The polyimides were derived from an oligomeric poly(amic acid) precursor [pyromellitic dianhydride (PMDA) and 4,4’-oxydianiline (ODA)I, a high molecular weight poly(amic acid ester) precursor (PMDA m-diacyl chloride diethyl ester and ODA), and a polyisoimide oligomer [3,3’,4,4’-benzophenonetetracarboxylic dianhydride (BTDA) and 1,3-bis(3-aminophenoxy) benzene (APB) endcapped with (3-aminophenyl) acetylene (APA)]. The number of "defect sites" or imide-precursor groups where imidization does not occur was estimated to be between 6 and 9% of the total nitrogen and varies with the type of precursor used. The degree of imidization or cure was found to vary between 91 and 94% following a cure at 400 degrees C. Residual isoimide groups were detected after an extended 400 degrees C bake of the polyisoimide precursor. Cured films were subjected to temperature and humidity stress at 85 degrees C and 81% relative humidity for 450 h. Estimates of hydrolysis range from as little as 1% of total nitrogen for the BTDA-APB-APA derived material to approximately 13% for the PMDA-ODA poly(amic acid ester) precursor. About 30% of the amide acid groups formed during stress react with water in a second hydrolysis reaction with chain cleavage to yield a terminal diacid and a terminal amine group. Hydrolysis from temperature and humidity stress is almost completely reversed if the stressed polyimide is heated at 400 degrees C after stress. The data obtained in this study are consistent with previously reported macroscopic observations in which polymer properties degrade during temperature and humidity stress and are recovered after post temperature and humidity bakes.