The mechanical behaviour of a styrene-butadiene rubber, containing 23.5% styrene and filled with up to 55% by weight with silica nanoparticles, has been examined. The tan 5 spectra of these composites show a second relaxation of relatively low intensity located about 40 degreesC above the main a relaxation, the glass transition T-g, which occurs at -34.5 degreesC in the unfilled polymer. This is attributed to an interfacial layer of polymer molecules whose chain relaxation dynamics have been altered by interaction with the filler surface. This is accompanied by a shift of the T-g itself to lower temperature when the filler surface is organophilic, but not when the filler surface is hydrophilic, and possible mechanisms for this are discussed. The amount of interfacial polymer can be quantified by resolving the glass transition and the second relaxation using a curve-fitting approach. A first-order treatment is presented which estimates the layer thickness to be of the order of a few nanometers. (C) 2003 Published by Elsevier Ltd.