Moisture effects on glass-reinforced-plastics have been investigated using a stress-rupture approach, Stress-rupture data for a powder- bonded E-glass chopped strand mat/polyester laminate was obtained for the following environments; 1) 5% RH; 2) laboratory conditions; 3) Water at room temperature; 4) water at 40°C; 5) IN hydrochloric acid Stress-rupture data for an emulsion bonded E-glass chopped strand mat/polyester laminate was obtained for laboratory conditions and water at room temperature. The effects of loading histories and porosities of up to 3% were also investigated. Failure mechanisms were investigated by scanning electron microscopic examination of the fracture surfaces, residual strength measurements and optical examinations before and after a resin burn off. Three failure mechanisms were shown to operate over the test period. The most common was caused by environmental stress corrosion of essentially pristine fibre lengths. In water at 40°C an additional strength loss was caused by interfacial degradation. The emulsion bonded mat laminate in water at room temperature exhibited a steeper stress-rupture curve due to the increased permeability of the matrix in the interfilament channels Surface damage was also noted for all the aqueous environments. The duration of the tests was, however, too short for the damage mode to influence the stress-rupture curves.