Layered-silicate nanoparticles, when distributed within the matrix of a fiber-reinforced polymer (FRP), can retard the diffusion of environmental moisture and other chemicals to the fiber-matrix interface where their presence can result in delamination and fiber weakening. Thus, the use of nanoparticles helps to preserve the integrity of FRPs and to prolong the service life of composites when these are used in outdoor applications such as bridges and utility poles. In the present work, diffusion coefficients of water, brine, and alkali were determined at several different temperatures by soaking clay/vinyl ester nanocomposites in the different liquids and measuring the increase in weight with increasing time of immersion. It was found that the measured diffusivities decreased with increasing clay content. However, the reduction in water diffusion coefficient was found to be less severe when steady permeation experiments were carried out. This result is theoretically explained based on the hydrophilic nature of the clay surface that tends to immobilize some of the moisture. (c) 2005 American Institute of Chemical Engineers.