Some mechanical properties of blends of polystyrene (PS) and ethylene-propylene-rubber (EP) were derived from stress-strain and impact measurements. The strength and impact properties are improved by adding EP-g-PS graftcopolymer, prepared by reacting PS with EP, to the blends. It is assumed that the EP-g-PS graftcopolymer acts as an adhesive at the interface between the thermoplast and the rubber phases. The addition of the graftcopolymer reduces the dimensions of the dispersed rubber particles. High values of impact strength at reasonable values of tensile moduli could be reached by replacing EP for a smaller or larger part by EP-g-PS copolymer. These kinds of EP-modified PS blends had much higher impact values than those of comparable PS blends containing low density polyethylene (I dPE) and 1 dPE-g-PS graftcopoly mer or this graftcopolymer only. It seems attractive to ascribe these results to the noncrystallinity of the PE-g-PS as compared with the crystallinity of ldPE in ldPE and in ldPE-g-PS. However some caution seems recommendable as EP-modified PS fractures with microshear whereas, the PE-modified PS shows crazing.