Vinylester/epoxy (VE/EP)-based thermosets of interpenetrating network (IPN) structures were produced by using a VE resin (bismethacryloxy derivative of a bisphenol A-type EP resin) with aliphatic (Al-EP) and cycloaliphatic (Cal-EP) EP resins. Curing of the EP resins occurred either with an aliphatic (Al-Am) or cycloaliphatic diamine compound (Cal-Am). Dynamic mechanical thermal analysis (DMTA) and atomic force microscopy (AFM) suggested the presence of an interpenetrating network (IPN) in the resulting thermosets. Fracture toughness (K-c) and fracture energy (G(c)) were used as the toughness characterization parameters of the linear elastic fracture mechanics. Unexpectedly high K-c and G(c) data were found for the systems containing cyclohexylene units in the EP network, such as VE/Al-EP+Cal-Am and VE/Cal-EP+Al-Am. This was attributed to the beneficial effects of the conformational changes of the cyclohexylene linkages (chair/boat), which were closely analogous to those in some thermoplastic copolyesters. The failure mode of the VE/EP thermoset combinations was studied in scanning electron microscopy (SEM) and discussed. (C) 2003 Wiley Periodicals, lnc.