Abstract 1. Conventional silica-reinforced NBR vulcanizates are characterized by stiffness at low elongation and low modulus at high elongations. These properties are associated with reticulate structure formation of silica particles, which occurs rapidly when compounds are heated to curing temperatures. 2. An NBR terpolymer, functionalized with about 0.07 mol of epoxy groups per kg polymer, interacts with silica when mixtures are subjected to combined heat and shear. Vulcanizate stiffness is greatly reduced, modulus at higher extensions is greatly increased, and mechanical properties in general are changed in the directions characteristic of carbon black reinforcement. 3. The interaction is similar in effect, but in principle much more economic, compared to that obtained with a mercaptosilane coupling agent. 4. Early experience indicates that the improvements are attainable to a large degree, although not optimally, in a conventional laboratory Banbury mixing operation.