The dynamic mechanical properties of a series of high impact polystyrene (HIPS) with a rubber phase based on styrene and butadiene (SBR) were studied. The variation of E' and tan delta as a function of initiator, chain transfer agent (CTA), and SBR concentration, as well as the polybutadiene (PB) content in SBR, during the synthesis of HIPS were evaluated. For the range of concentrations studied, it was found that E' decreased with an increase in the initiator and/or CTA concentration, as well as with an increase in the SBR content in HIPS and PB content in SBR, independently of the type of rubber phase particle obtained. In the case of tan delta, it showed a minor peak at -80 +/- 10 degrees C, which decreased in magnitude and shifted to lower temperatures with the initiator concentration, but increased with an increase in the PB content in the rubber phase. In addition, transmission electron microscopy (TEM) showed that in most cases the particle size of the rubber phase was between 0.15 and 0.25 mu m, except for those HIPS samples that contained SBR with 90 wt% of PB, in which it was much larger (0.45-1.15 mu m). In the latter case it was found that the modulus decreased with the PB content, and decreased further and perceptibly with increasing particle size.