Investigations of electron beam (e-beam) and ultra violet (UV)-induced cationic polymerization kinetics of a mono-functional epoxy system, phenyl glycidyl ether (PGE), were conducted using real time in situ near infrared (NIR) spectroscopy. Effects of processing variables such as temperature and dose rate on initiation and propagation rate constants have been assessed. The experimental results and results from a simple mathematical model developed to predict the reaction behavior under continuous irradiation showed very good agreement. This work provides a basis for investigating the cure behavior of more complex and industrially relevant crosslinking epoxy systems where diffusion limitations play an important role as discussed in PART II. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013