The accelerated effects of immersion in seawater at 60 degrees C, in comparison to distilled water, on a vinyl ester resin and on its highly quartz particles filled system were studied. The studied resin was cured at room temperature for 48 h and postcured at 100 degrees C for 12 h, and the filled system contained 93 wt % quartz particles of a wide size distribution, either as received or silane-treated. The immersion in water results mainly in further curing and some extraction of low molecular substances, as reflected by the flexural mechanical properties and the DSC thermograms. The silane coupling agent treatment of the filler significantly improves che composite properties and its resistance to the water environment. No practical difference was observed in any of the studied systems between the effects of sea and distilled water, as tested by the immersion in boiling water for 48 h. Weatherometer conditions, cycles of ultraviolet radiation at 60 degrees C and water condensate at 50 degrees C, for up to 1000 h, also cause postcuring effects, without extractables＇ leaching effects.