The imidization and phenylethynyl end-group reaction behavior of phenylethynyl-terminated imide oligomers (LaRC(TM) PETI-6) with a number-average molecular weight of 2500 g/mol has been extensively interpreted in terms of the disappearance and appearance of several characteristic absorption peaks. Most characteristic FTIR absorption bands including the phenylethynyl end group occurring via imidization and cure during dynamic and isothermal heat treatments of LaRC(TM) PETI-6 were clearly resolved and assigned in the present study. The results give some information on temperature- and time-dependency of the cure of LaRC(TM) PETI-5, which may be useful for determining the processing conditions necessary for use as a high-temperature fiber-sizing material. This study also measures the extent of imidization and the activation energy of cure reaction of this material. (C) 2000 John Wiley & Sons, Inc.