We analyzed the effects of clay layers on the barrier properties of polymer/clay nanocomposites containing impermeable and oriented clay layers. Using the relative permeability theory in combination with the detour theory, we obtained new relative permeability expressions that allow us to investigate the relative permeability R-p as a function of lateral separation b, layer thickness w, gallery height H, layer length L, and layer volume fraction Phi(s). It was found that intercalated and/or incomplete exfoliated structures and dispersed tactoids with several layers can effectively enhance the barrier properties of the materials Furthermore, we developed the chain-segment immobility factor to briefly discuss the chain confinement from clay layers. The results showed that the chain confinement enhanced the barrier properties of the intercalated nanocomposites. Our model is better consistent with the experiments when Phi(s)> 0.01. The findings provide guidelines for tailoring clay layer length, volume fraction and dispersion for fabricating polymer-clay nanocomposite with the unique barrier properties. (c) 2006 Elsevier Ltd. All rights reserved.