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Glass fiber reinforced composites have been used to an increasing extent as structural components because of their high strength at relatively low densities. They are more resistant to moisture and weathering than most other structural materials. However, under environments of high humidity and temperature, glass fiber composites with an epoxy or polyester matrix lose a considerable fraction of their strength after a relatively short time. For high strength glass reinforced composites, the glass filaments are mostly of the S-glass and E-glass types. This investigation reports the solvent effect of water on S-glass rovings (filament strands) and powders. S-glass is the strongest silicate glass developed in the past decade, the composition of which in weight percent is: Al,O:,/MgO/SiO, = 25110165. In the temperature range covered (30 to 50°C), analyses ofaluminum, magnesium, and silicon in water after different time periods indicate that magnesium is preferentially leached out of S-glass, either as filaments or as powders, h y water. T h e concentrations of magnesium and silicon in the water reached 2 mgilitre or more depending on the temperature and length of contact time, while the aluminum concentration reached no more than 0.2 mgilitre. Decomposition products of the ingredients present in a composite, including the coupling and curing agents, could further complicate the degradation process. Rate relations are proposed and discussed based on the dissolution data.