It is proposed that, when vulcanization is performed using peroxides, crosslinking leads to a simple network, whereas in conventional vulcanization crosslinking a partially interpenetrating polymer network (PIPN) is formed. Two unfilled polyisoprene networks of similar crosslink density, produced with dicumyl peroxide and 2-bisbenzothiazole-2,2'-disulfide/ sulfur formulations, were compared with respect to the effect of strain rate on their stress-strain and hysteresis curves at room and elevated temperatures. At high elongations, the stress-strain curves for peroxide vulcanizates show a steeper upturn than for conventional vulcanizates, but have lower tensile strength and elongation at break. On increasing the extension rate, stress-strain curves for peroxide vulcanizates rise less steeply, while conventional vulcanizates rise more steeply. For both vulcanizates the hysteresis ratio decreases on increasing the rate at which samples are extended and retracted. The effect on conventional vulcanizates is less than on peroxide vulcanizates. It is suggested that chains in peroxide networks disengage increasingly rapidly at higher strains, allowing increased strain-induced crystallization. Rapid strain-induced crystallization leads to low ultimate tensile strength (UTS). In more complex PlPNs, the disengagement and alignment of chains are retarded. The increased nonuniform extension of chains promotes early strain-induced crystallization at low extensions, but overall it reduces the rate of crystallization, which occurs over a wider range of strains. This improves UTS and elongation. (c) 2006 Wiley Periodicals, Inc.