Detailed atomistic structures of blend (class I) and chemical tethered (class II) poly vinyl alcohol (PVA)-silica hybrid membranes were constructed to investigate the diffusion behavior of small molecules at the interfacial region. Stronger interaction between PVA and silica, as well as more appropriate interfacial morphology which was evaluated by interchain distance, chain mobility and free volume were found in class II hybrid membranes due to the presence of covalent bonds. The effects of interfacial morphology on diffusion behavior of small molecules including benzene and cyclohexane were tentatively explored, and found that diffusion coefficients were closely related to fraction of free volume (FFV). Subsequently, the diffusion selectivity was calculated by the ratio of FFV probed by benzene and cyclohexane molecules. Hopefully, this study will offer some important qualitative insight into the transport phenomena within organic-inorganic hybrid membranes. (c) 2007 Elsevier Ltd. All rights reserved.