A maleimido-terminated butadiene-acrylonitrile copolymer was employed as an impact modifier for a commercial thermosetting bismaleimide resin. FTIR spectroscopy was used to monitor in real time the kinetics of the curing process in this blend system at different temperatures. The toughening agent was found to produce strong effects on the kinetics and mechanism of the curing process. The fracture toughness parameters K-c and G(c) showed a substantial enhancement as the rubber content in the blend was increased. Only a modest reduction of the elastic modulus and of the compressive yield stress was brought about by incorporation of the impact modifier.