Stress softening of vulcanizates of SBR 1500 containing different blacks possessing the same “structure” but varying in surface activity, and effects of different black loadings, of black structure levels, and of particle size, were investigated. It was concluded: 1. Strain-energy loss can be used as a quantitative measure of stress softening, and initial strain-energy input as a measure of prestress severity. 2. The effects of carbon black and polymer variables can be normalized in a single general relationship by plotting per cent strain-energy loss as a function of initial strain-energy input for filled vulcanizates. 3. With the exception of natural rubber, gum vulcanizates studied showed no stress softening. The stress softening of natural rubber gum vulcanizates is attributed mainly to stress-crystallization. 4. Stress-softening of filled vulcanizates is not a completely reversible process. Rates of stress recovery are reasonably rapid. 5. The degree of stress softening can be predicted from the initial stress—strain curve, the prestress severity desired, and the general correlation based on strain-energy considerations found in this study. 6. Prestressing reduces abrasion resistance as measured in the laboratory.