The diffusion coefficients of water vapor in poly(vinyl alcohol)-fumed silica (PVA-FS) nano-composite membranes were determined using the gravimetric method. Water vapor was observed to diffuse more rapidly in membranes with increased FS content. The vapor diffusion coefficient was determined as 1.2 x 10(13) m(2)/s in pure PVA and was observed to increase to 3.0 x 10(-13) m(2)/s in PVA composites containing 30% FS nano-particles. The free-volumes of PVA-FS membranes were characterized using positron annihilation lifetime (PAL) spectroscopy. PAL results showed that both the ortho-positronium (o-Ps) life-time and intensity increased with the addition of FS. The intensity (I-3) was found to be higher than the estimated value determined from the linear combination of the data from pristine PVA and FS, and correlated excellently with the polymer amorphous content. The PAL results indicate that a higher FS content in PVA increases the free-volume hole size(a volume increase from 40 to 55 angstrom(3)) and free-volume hole density (an 13 increase from 23 to 28%), resulting in a higher fractional free-volume in the nano-composites. The increase in the relative polymer free-volume with higher FS content was associated with a decrease in the PVA crystallinity, as determined from differential scanning calorimetry measurements. It is postulated that the incorporated FS nano-particles interrupt polymeric chain packing and retard crystallization during membrane formation. More crystalline segments were transformed into amorphous regimes in the nano-composites containing more FS. A correlation between water diffusivity and the fractional free-volume was obtained, and the water diffusivity was successfully expressed by the free-volume theory. (C) 2008 Elsevier B.V. All rights reserved.