Diffusion of seawater in unsaturated polyester resin (UPR) and its glass fiber reinforced composite in the presence of titanium dioxide has been studied by sorption method. Incorporation of glass and TiO2 to UPR alters the seawater diffusion process from Fickian to non-Fickian type. The dual mode sorption model is used to separate Fickian-controlled and relaxation-controlled diffusion in case of UPR-T, UPR-G, and UPR-GT. The presence of TiO2 seems to stabilize UPR and its glass reinforced samples but an increase in the rate of seawater diffusion is observed for these systems. The free volume determined from positron lifetime measurements support the diffusion data in these systems. Results further indicate that the contribution to diffusion in the later stages of sorption is due to the increased contribution from the interfaces. The plasticizing effect of TiO2 is clearly seen even in the glass reinforced composite. DMA results show an increase in flexibility because of TiO2 presence both in the neat as well as glass reinforced resin which is well supported by decrease in T-g value from DSC data. (c) 2006 Wiley Periodicals, Inc.