An experimental programme is presented for the effect of moisture on the toughness, mode-I and mode-II of aluminium tri-hydrate and polyethylene filled and unfilled quasi-isotropic glass–fibre reinforced epoxy–vinylester resin (GFRP) composites. Specimens were exposed in water at room temperature (20°C) for a period of 8 months and the effect of moisture content on toughness, GIc and GIIc values were obtained at an interval of every 2 months. Some samples were exposed in hot water at 40°C temperature to accelerate the uptake of moisture and produce saturated composites. The results indicate that equilibrium moisture content and diffusion coefficients increase with increase of weight of filler content in GFRP composites which is linked to an increase in microscopic cracking. Also mode-I, toughness of all composites increased with an increase in moisture uptake, mode-II toughness was relatively unaffected. Aluminium-tri-hydrate filled GFRP composites showed a higher moisture uptake, which resulted in higher values of both mode-I and mode-II, toughness than the polyethylene filled and unfilled GFRP composites.