The polymerisation kinetics for IPNs formed from a model vinyl ester resin (VER) and an imidazole-cured epoxy resin (DGEBA) has been studied by scanning DSC and isothermal FTIR. The chemical interactions between the VER initiating system (either cumene hydroperoxide, CHP; methyl ethyl ketone peroxide, MEKP; benzoyl peroxide, BPO; or azobisisobutyronitrile, AIBN) and the epoxy curative (l-methyl imidazole, 1-Mel) have been examined. IPNs containing AIBN showed minimal initiator interactions but a strong accelerating effect was observed by the imidazole curing agent on the peroxide initiator systems (at least in the early stages) due to enhanced radical production apparently via a redox reaction. For IPNs containing MEKP and BPO, the imidazole also appeared to cause a significant loss of initiator activity, perhaps by non-radical redox side reactions. During the isothermal cure of the IPN, the DGEBA monomer acted as a diluent on the polymerisation of the ate-initiated Vinyl ester, resulting in a slower initial cure rate than in the neat VER. Due to the slower cure of the 1-Mel/DGEBA component, the DGEBA monomer acted as a plasticiser in the later stages of the isothermal VER cure, thus enabling higher final levels of vinyl cure prior to vitrification. As a consequence, the higher level of crosslinking of the VER component in the IPN reduced the maximum isothermal conversion of the epoxy component during its isothermal cure. (C) 2001 Elsevier Science Ltd. All rights reserved.