Novel organic-inorganic hybrid membranes composed of poly(vinyl alcohol) (PVA) and gamma-(glycidyloxypropyl)trimethoxysilane (GPTMS) were prepared through in situ sol-gel approach. The chemical structure and reaction mechanism for PVA-GPTMS hybrid membranes were tentatively proposed based on FTIR and solid-state Si-29 NMR spectra. The free volume characteristics of PVA-GPTMS hybrid membranes were revealed by positron annihilation lifetime spectroscopy (PALS), and the apparent fractional free volume (f(app)) were calculated according to free volume cavity size and intensity of o-Ps. The unusual pervaporation properties of PVA-GPTMS hybrid membranes for separating benzene/cyclohexane mixtures were discovered. The permeation flux increased from 20.3 g/(m(2)h) for GPTMS-free membranes to 137.1 g/(m(2)h) for PVA-GPTMS-04 membrane, while separation factor increased from 9.6 to 46.9 simultaneously. The correlation between f(app) and permeation flux was established and the improvement in permeability reflected hybridization-induced disruption of polymer chain packing and an accompanying elevated free volume available for molecular diffusion. (c) 2005 Elsevier B.V. All rights reserved.