The measurements of free-volume hole property by positron annihilation lifetime spectroscopy (PALS) have been carried out for polymer-clay nanocomposite materials that consist of styrene-butadiene rubber (SBR) and layered silicate clay of rectorite and conventional composite materials N326 (carbon black) /SBR. The PALS and differential scanning calorimeter (DSC) results show layered rectorite has a stronger effect on restraining polymer chain mobility which results in the decrease of fraction free volume and gas permeation than carbon black. The dispersion of nanoscale rectorite clay in SBR largely enhances gas barrier property in contrast to results obtained in N326/SBR system. Experimental results reveal that gas permeability in rectorite/SBR nanocomposite is mainly influenced by fractional free volume and tortuous diffusional path effects attributed to the clay plateletlike morphology. (C) 2004 Elsevier Ltd. All rights reserved.