The structure and properties of copolymerized sequential-interpenetrating networks (SeqIPNs) synthesized from amine-cured epoxies and free-radical polymerized dimethacrylates were examined. Materials were synthesized with and without the incorporation of an epoxy-terminated butadiene-nitrile reactive elastomer. Synthesis proceeded through full thermal cure of the epoxy-amine network, followed by polymerization of the methacrylate network. The methacrylate reactions were free-radically induced using thermal (peroxide-initiated) or photochemical [electron-beam (e-beam)] techniques. Fourier transform infrared spectroscopy was used to monitor epoxy-amine step-growth polymerization in situ and to measure final cure conversion of methacrylates. Structural examination of the IPNs using atomic force microscopy and scanning electron microscopy revealed microphase separation in the neat-SeqIPN materials and macroscopic phase separation of rubber-rich domains for elastomer-modified networks. Dynamic mechanical analysis of the SeqIPN determined that the properties of the network are strongly dependent on the cure conditions. Furthermore, the viscoelastic behavior of the e-beam-cured SeqIPN could be adequately described by the Williams-Landel-Ferry and Kohrausch-Willams-Watts equations, presumably because of a strong coupling between the epoxy-amine and methacrylate networks.