In this article, we describe a method used to prepare an in situ sodium-activated, organomodified bentonite clay/styrene-butadiene rubber nanocomposite master batch via a latex blending technique. The clay master batch was used for compound formulation. Octadecyl amine was used as an organic intercalate. The clay was purchased from local suppliers and was very cheap. Sodium chloride was used for in situ activation of the clay. The wide-angle X-ray diffraction data indicated that the in situ sodium activation helped to increase the intergallery distance from 1.28 to 1.88 nm. A transmission electron micrograph indicated intercalation and partial exfoliation. The thermal properties were relatively better in the case of the sodium-activated, organomodified bentonite-clay-containing compound. A substantial improvement in physical properties such as the modulus, tensile strength, tear strength, and elongation at break was observed in the case of the in situ sodium-activated compound. A cationexchange capacity equivalent (of the clay) of 1.5 times the octadecyl amine was the optimum dose for the modification. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 1316-1329, 2009