IPNs based on polyethylene and vinyl polymers were prepared according to a new procedure. We studied the dynamic mechanical behavior of two series of IPNs: polystyrene (PS)-polyethylene (PE) and poly(butyl methacrylate) (PBMA)-PE at the frequency ω = 1 Hz and in the temperature region from −50 to 200°C. Temperature dependences of the components of the dynamic modulus of elasticity G' and G" of the networks PS and PBMA have shapes typical of amorphous networks; the corresponding dependences of pure PE show, however, features typical of a semicrystalline polymer. IPNs of the system PS/PE show two-phase behavior. AtT< 110°C, PE functions as a plasticizer. In the system PBMA/PE, better miscibility of components is seen, and PE exerts a reinforcing effect on the mechanical behavior over the whole temperature region. In both systems, network density increases with increasing PE content in IPNs. Better homogeneity and a slight increase in the network density of IPNs with PBMA/PE in comparison to PS/PE networks are probably caused by a greater number of grafted PBMA chains in the PE network compared to the PS network.