The reinforcing effect of a large amount of synthetic precipitated amorphous white silica nanofiller on the mechanical properties of styrene-butadiene rubber was studied. The silica surfaces were pretreated with bis(3-triethoxysilylpropyl)tetrasulfane (TESPT). TESPT is a bifunctional organosilane that chemically adheres silica to rubber and also prevents silica from interfering with the reaction mechanism of sulfur cure in the rubber. The silica particles were fully dispersed in the rubber and the chemical bonding between the rubber and filler was optimized by the incorporation of accelerator and activator in the rubber. This study showed that the mechanical properties of the rubber vulcanizate improved substantially when the filler was added. The addition of elemental sulfur affected the rubber properties, although there was no overall advantage, as some properties improved and others deteriorated. (c) 2007 Wiley Periodicals, Inc.