The incorporation of fillers into elastomers has profound effects on the mechanical, physical, and thermal properties of the nanocomposites that form. In this study, styrene-butadiene rubber as a matrix was reinforced separately with 10-, 15-, or 23-nm CaSO4, which was synthesized by an in situ deposition technique. The mixing and compounding were performed on a two-roll mill, and sheets were prepared in a compression-molding machine. Properties such as the swelling index, specific gravity, tensile strength, elongation at break, modulus at 300% elongation, Young's modulus, hardness, and abrasion resistance were measured. The morphology of the rubber nanocomposites was also performed with scanning electron microscopy to study the dispersion of the nanofiller in the rubber matrix. The thermal decomposition of the rubber nanocomposites was studied with thermogravimetric analysis, and the results were compared with those of commercial CaSO4-filled styrene-butadiene rubber. A reduction in the nanosizes of CaSO4 led to an enhancement of the mechanical, physical, and thermal properties of the rubber nanocomposites. Above a 10 wt % filler loading, the styrene-butadiene rubber showed a reduction in all properties. This effect was observed because of the agglomeration of the nanoparticles in the rubber matrix. The thermodynamic parameters were also studied. (c) 2007 Wiley Periodicals, Inc.