The cure mechanism of a trifunctional epoxy resin with an amine catalyst has been studied using near infra-red spectroscopy. The concentrations of primary and secondary amine and epoxide groups have been monitored directly as a function of cure by the use of this technique. This was compared to thermal analysis of the cure process at a range of cure temperatures by d.s.c. The conversion of the various functional groups and the increase in the glass transition temperature was found to be uniquely determined by the degree of cure and unaffected by cure temperature. The primary amines were found to be largely reacted by gelation, followed by the commencement of the secondary amine reaction. By monitoring hydroxyl production it was possible to determine the degree to which side reactions (reactions other than the main epoxy/amine reaction) occurred. These side reactions, such as etherification, occurred at conversion values above gelation and become prominent at high levels of crosslinking when the reactive functionalities become strongly diffusion controlled and their movement sterically hindered. At these high conversions unreacted secondary amines become trapped in the glassy network and these groups may result in disadvantageous properties of the final material.