The effect of pressure on the radiation-induced cross-linking of synthetic rubbers, cis-l,4-polybutadiene, cis-l,4-polyisoprene, poly(TFE-propylene), poly(styrene-butadiene), poly(ethylene-propylene), and trans-l,4-polybutadiene were studied. A remarkable acceleration of cross-linking by compression for unsaturated polymers is attributed to an acceleration of the propagation reaction of cross-linking, which does not require long-range migration of radicals, by reduction of the distance between polymer chains at high pressure while the termination reaction, which requires long-range migration of radicals to recombine with each other, is reduced by the decrease in mole-cular mobility at high pressure. The presence of an optimum pressure was observed for radiation-induced cross-linking of cis-l,4-polyisoprene and TFE-propylene copolymer. This is explained by a significant change in molecular motion occurring due to the glass transition or crystallization at high pressure.