Abstract Two petroleum pitch products have been shown to impart greater stiffness and hardness, and lower rebound properties to SBR and SBR/BR vulcanizate systems, through unusual molecular interaction effects. These solid pitch materials can be used alone as plasticizers or in combination with conventional aromatic extender oils. Vulcanizates containing these materials at 10–25 per cent concentrations maintain enhanced stiffness and hardness at temperatures to 150° C. Plasticizer evaporative loss and migration from higher to lower concentration areas is much less in pitch-containing vulcanizates than in stocks containing usual aromatic extender oils. Vulcanizate physical properties are presented. An immersional microcalorimetry study was conducted to investigate the energetics of interaction of carbon black with pitch in comparison with other major components of the elastomeric matrix. The thermodynamic measurements, which indicate specific pitch—carbon black interactions, provide much insight into the mechanisms responsible for the unusual properties which pitch imparts to rubber systems.