The moisture diffusion and barrier characteristics of epoxy-based nanocomposites containing organoclay are investigated. The effects of different types of organoclay modified with different compatibilizers, including a quaternary alkylamine-modified montmorillonite (KH-MT), a quaternary ammonium-modified montmorillonite (Cloisite 20A) and an octadecylamine-modified montmorillonite (I30P), on moisture barrier and thermomechanical properties are specifically studied. The moisture absorption and diffusion behaviours were different depending on the type of organoclay: the moisture absorption rates of the Cloisite and I30P systems were much lower with associated lower diffusivity than the KH-MT system or the neat epoxy, due to the higher interlayer distance and more uniform distribution of organoclays, which in turn allowed a longer diffusion path of water molecules in the nanocomposite. The moisture diffusivity of nanocomposites decreased with increasing clay content for all organoclays. The corresponding moisture permeability was lower in the order of nanocomposites containing I30P, Cloisite and KH-MT organoclays. The moisture permeability showed a systematic decrease with increasing clay content, which agrees well with the prediction based on the simple tortuous path model. Increase in effective penetration path due to the very large aspect ratio of the silicate layers was responsible for the reduced moisture permeability. The glass transition temperature was much higher for the I30P nanocomposite than the neat epoxy resin, whether dry or wet condition. It decreased linearly with increasing moisture content. The CTE decreased with increasing clay content for all conditions studied. (c) 2004 Elsevier Ltd. All rights reserved.