Stress relaxation on cessation of extension and hysteresis were examined in conventional and peroxide vulcanizates over a range of crosslink densities at room temperature and at 90 degrees C. The rate of relaxation decreases with an increase in temperature and is attributed to slower nucleation of strain-induced crystallites. The decrease in the volume fraction of extendable material as a result of strain-induced crystallites has only a small effect on the rate at which the slope of the stress-strain curve rises. Crystallites that form on cooling, melt on heating, leaving an unaltered network, but when. strain-induced crystallites are melted by heating, they do not reform on cooling. The relaxed network now extends further before failure than a network in which extension was not interrupted by a heating-cooling cycle. This supports proposals in the preceding article that strain-induced crystals increase tensile properties by altering the network deformation pattern and not by the deflection of propagating flaws, whose mechanism would require the heated sample to fail earlier due to its lower crystalline content. Hysteresis increases sharply at strains at which crystallization in the network becomes possible. The hysteresis ratio reaches a plateau value at higher strains. Relaxation affects both the extension and retraction stress-strain curves and it is proposed that hysteresis is determined by differences in the crystallinity during extension and retraction. (c) 2006 Wiley Periodicals, Inc.