The kinetic mechanism of the thermal cure of a phenylethynyl-terminated imide model compound, 3,4-bis[(4-phenylethynyl)phthalimido]diphenyl ether (PEPA-3,4-ODA) and a phenylethynyl-terminated imide oligomer PETI-5 (MW 5000 g/mol) was studied. FTIR was used to follow the cure of the model compound, while thermal analyses (DSC) was used to follow the cure of the PETI-5 oligomer. The changes in IR absorbance of phenylethynyl triple bonds at 2214 cm 01 of PEPA-3,4-ODA as a function of cure time were detected at 318, 336, 355, and 373ΠC, respectively. The changes in the glass transition temperature, T g , of PETI-5 as a function of time were measured at 350, 360, 370, 380, and 390ΠC, respectively. The DiBenedetto equation was applied to define the relative extent of cure, x, of the PETI-5 oligomer by T g . For the model compound, the reaction followed first order kinetics, yielding an activation energy of 40.7 kcal/mol as determined by infrared spectroscopy. For PETI-5, the reaction followed 1.5th order, yielding an activation energy of 33.8 kcal/mol for the whole cure reaction, as determined by T g using the DiBenedetto method. However, the cure process of PETI-5 just below 90% by this method followed firstorder kinetics yielding an activation energy of 37.2 kcal/mol.