The conventional gravimetric method and positron lifetime spectroscopy have been used to investigate the effect of glass fiber orientation on the diffusion behavior of seawater in epoxy-based composite samples with glass fiber orientations of 0 and 45 degrees. The equilibrium mass uptake of seawater in 45 and 0 degrees orientation composites has been found to be 2.77 and 1.57%, respectively. The diffusion process is non-Fickian in a 45 degrees fiber oriented composite, whereas it is Fickian in a 0 degrees oriented composite. Free-volume data for 45 degrees fiber oriented composites indicates swelling upon the sorption of seawater leading to structural relaxation, and hence the diffusion becomes non-Fickian. On the other hand, a 0 degrees fiber orientation sample exhibits no swelling, and this suggests that water diffusion to the fiber-resin interface through the resin matrix is impeded by the large number of bonds. A polymer-fiber interaction parameter determined from these results also further supports the idea that interface interaction in a 45 degrees fiber oriented composite is less than that in a 0 degrees fiber oriented composite. Positron and gravimetric results support this argument. (C) 2008 Wiley Periodicals, Inc.