A polystyrene-modified natural rubber (SNR) with 80% degree of grafting was evaluated against natural rubber (NR) in their blends with polystyrene (PS). The rubber loading of the PS-SNR and PS-NR blends was varied from 5 to 20% by volume. At 10-15% rubber loading, the PS-SNR blends were found to be approximately 8-10% higher in tensile strength and 7-13% higher in Young's modulus than the PS-NR blends. Over the range of rubber loading investigated, it was also observed that the PS-SNR blends were 5-42 and 14-36% higher in flexural strength and flexural modulus, respectively. The most pronounced difference between the two blend systems is in their impact strength, where the former is about 55-230% higher than the latter. Relative to the pure PS, the PS-NR and PS-SNR blends are approximately 50-250 and 140-1050% higher, respectively, in impact strength. Morphological observations, which are consistent with the relative tensile, flexural, and impact properties of the two systems, indicate that SNR is more compatible with PS than NR and more homogeneously dispersed in the PS matrix. (C) 2004 Wiley Periodicals, Inc.