The curing behaviour for an Interpenetrating Polymer Network (IPN) formed from a model Vinyl ester resin (VER) and an amine cured epoxy resin has been studied by scanning DSC and isothermal mid-FTIR. The interactions between the VER initiating system (azobisiso-butyronitrile or cumene hydroperoxide, benzoyl peroxide or methyl ethyl ketone peroxide/cobalt octoate) and the epoxy curative (aniline, diaminodiphenyl methane, butylamine or diamino-octane) have been examined. For most of the IPNs, there was evidence for a reduction in reaction rate due to the dilution of each reacting system by the other resin components. When cumene hydroperoxide, benzoyl peroxide or methyl ethyl ketone peroxide (with or without cobalt octoate) were used as the radical initiating systems, there was strong redox interaction between the peroxide and the amine, which caused acceleration of the peroxide decomposition in the early stages of reaction and also resulted in premature depletion of the initiator system. Evidence for a grafting reaction between the amines and the methacrylate groups by Michael addition was also found. In some systems cured isothermally at 70 degreesC, the slow cure of the epoxy component allowed the unreacted DGEBA to plasticise the system and enhance the extent of cure of the VER components prior to vitrification of the IPN. As a consequence of the additional crosslinks introduced by the VER component, premature vitrification occurred during the slower cure of the DGEBA component, thus reducing the extent of cure of the epoxy groups in the IPN. Near full cure could be obtained for most of the IPN systems when post-cured at elevated temperature, indicating that the presence of each network imposed minimal topological restrictions on cure.