We studied the stress-relaxation behavior of natural rubber (NR)/polystyrene (PS) blends in tension. The effects of strain level, composition, compatibilizer loading, and aging on the stress-relaxation behavior were investigated in detail. The dispersed/matrix phase morphology always showed a two-stage mechanism. On the other hand, the cocontinuos morphology showed a single-stage mechanism. The addition of a compatibilizer (NR-g-PS) into 50/50 blends changed the blend morphology to a matrix/dispersed phase structure. As a result, a two-step relaxation mechanism was found in the compatibilized blends. A three-stage mechanism was observed at very high loadings of the compatibilizer (above the critical micelle concentration), where the compatibilizer formed micelles in the continuous phase. The aged samples showed a two-stage relaxation mechanism. The rate of relaxation increased with strain levels. The aging produced interesting effects on the relaxation pattern. The rate of relaxation increased with temperature due to the degradation of the samples. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 108: 904-913,2008.