An unsaturated polyester resin has been modified by adding a thermosetting bismaleimide as a second co-reactive monomer. The bismaleimide resin was readily dissolved in the uncured polyester matrix up to a concentration of about 20 wt%. Fourier transform infra-red (FT i.r.) spectroscopy was employed to monitor the cure kinetics of this complex system. The available spectroscopic evidence indicated that both the mechanism and the kinetics of the crosslinking process were strongly affected by the presence of the bismaleimide in the system. The molecular structure of this intercrosslinked network is discussed. The dynamic mechanical behaviour of a typical blend composition was found to be in agreement with the proposed molecular structure.