Cardanol is the main ingredient of cashew nut shell liquid, an agro-byproduct. Its comparative merits vis-a-vis aromatic oil for plasticizing HAF-black-filled natural rubber (NR) is the subject of this article. Aromatic oil is the conventional plasticizer employed for such compounding requirements. The mechanical properties, ageing behavior, and torque time curves during cure of NR plasticized by both these materials are compared under identical conditions. It has been established that cardanol, when used as plasticizer, gives rise to similar tensile properties, shorter cure times, and superior ageing behavior compared to aromatic oil. (c) 2006 Wiley Periodicals, Inc.