New classes of hybrid polymer networks (HPNs), having variable polyurethane acrylate (PUA) and epoxy-amine acrylate (EAA) compositions, were prepared using initially miscible systems in methyl methacrylate (MMA). The initial systems were based on PUA prepolymer and EAA monomer solutions in MMA. HPNs were a result of epoxy-amine and radical polymerization competition. Phase separation occurred during the course of HPN formation. Mechanical dynamic analysis of the prepared HPNs showed good affinity between the PUA and PMMA phases and lower affinity between the EAA and PMMA phases. Mechanical property evolution and transmission electronic microscopy showed that, for all the composition ranges used in this study (PUA/EAA/PMMA 15/45/40-45/15/40 wt %), the PUA-rich phase was the continuous phase. EAA-rich phases, 20-50 nm, in the PUA-rich matrix were obtained for HPNs containing up to 30 wt % EAA. For higher EAA concentration (45 wt %), 2 mu m EAA-rich phases were obtained in the PUA-rich matrix. A substructure was also observed in each phase. PUA/EAA copolymers were prepared and used successfully for the compatibilization of the different phases of the HPNs.