Fully interpenetrating polymer networks (fully IPNs) based on diglycidyl ether of bisphenol A (DGEBA) and ethylene dimethacrylate (EDMA) were prepared using isophronediamine and benzoyl peroxide as curing agents. Semi-interpenetrating polymer networks (semi-IPNs) based on DGEBA and 2-hydroxyethyl methacrylate (HEMA) were also prepared using the same curing agents. The average activation energies of transition for both these IPNs were calculated from rheometric dynamic spectroscopy (r.d.s.) data, which indicated higher values for the fully IPNs than the corresponding semi-IPNs. The more acrylic the interpenetrating polymer network (both fully and semi-IPNs) contained the lower the activation energy of transition. Broadened tan 6 peaks were observed for compatible materials, implying an enhanced cracking energy absorption• RDS data also revealed a maximum half-width of tan 6 at 30% of EDMA for the fully IPNs and at 25% of HEMA for the semi-IPNs. Beyond these compositions, decreased impact resistance and lower tensile modulus and elongation values were observed for these materials, presumably because of microphase-separation.