Abstract The importance of different types of colloidal carbon as reinforcing agents for the butadiene-styrene copolymer, GR-S, has been stressed in recent papers. It has been shown that, to a first approximation, the effect of carbon blacks in this type of synthetic rubber is similar to that in natural rubber, but it has been shown also that the extremely low tensile strength and poor tearing properties of uncompounded vulcanized GR-S necessitates the addition of some form of carbon black to almost all types of compounds. The present paper considers the influence of carbon blacks in vulcanized GR-S compounds. Earlier papers have discussed the effect of carbon blacks in natural rubber.