Composites based on styrene-butadiene rubber containing organophilic montmorillonite were produced by melt compounding and conventional sulfur curing. The samples were characterized by X-ray diffraction and both transmission and scanning electron microscopy. The dispersion of the clay and the spacing between the silicate layers revealed the presence of intercalated, aggregated, and partially exfoliated structures. Infrared spectroscopy also provided clear evidence for clay exfoliation and migration of zinc stearate to the surface of the samples. The crosslink density, evaluated through swelling in toluene, decreased with increasing organoclay content. This behavior could be justified by the partial absorption of the curatives on the filler surface. The mechanical properties of nanocomposites significantly increased when compared with those of unfilled rubber. These enhanced properties were attributed to the intercalation/exfoliation of the organoclay. POLYM. ENG. SCI., 51:1720-1729, 2011. (C) 2011 Society of Plastics Engineers