AbstractClays belong to an economic class of fillers, which are used extensively in rubbers and plastics. Being nonreinforcing in nature, there are limitations upon its use. If the properties of filler are modified, it will get a higher value as a filler. To achieve this modification of surface properties is one of the avenues. In the present work, the effect of treatment of the coupling agent on clay has been studied, with polybutadiene as a matrix. Composites were made with a varying proportion of untreated and treated clay. A two‐roll mill was used for dispersing the filler in the rubber, and a compression‐molding technique was used to cure the compounded in sheet forms. Tensile properties were measured on a computerized UTM using the ASTM procedure. Comparison of properties of composites filled with treated and untreated clay established that treatment of clay imparts better reinforcing properties. The properties under consideration were tensile strength, modulus at 100 and 400%, Young's modulus, hardness, etc. Tensile strength was improved by 52%, while modulus at 400% was improved by 150%. Similarly Young's modulus also was improved by 27%. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1299–1304, 2004