Moisture diffusion was studied through vinyl ester samples containing up to 5 wt% of organically treated montmorillonite. Two different kinds of clay surface treatment were employed to make the clay compatible with Derakane(TM) 411-350 vinyl. ester resin. The nanocomposites were characterized using differential scanning calorimetry (DSC), mechanical property measurements, X-ray diffraction (XRD) and transmission electron microscopy (TEM). TEM pictures showed that the clay platelets were either exfoliated or intercalated, and the two different surface treatments resulted in different dispersion characteristics. All the samples were post cured, and the diffusivity of moisture was measured by soaking the samples in 25degreesC water and noting the increase in weight with increasing time of immersion. It was found that water diffusivity decreased with increasing clay content, and it was reduced to half its value in the neat resin when the clay content was only 1 wt% regardless of the nature of clay surface treatment. However, the equilibrium moisture content, the glass transition temperature, and the elastic modulus all increased with increasing amounts of clay.