Two investigations into the effects of environmental exposure on the mechanical properties of glass reinforced polymer composites are described. In the first, a glass/vinyl ester composite showed a 15% decrease in the tensile strength and a 6% increase in the tensile modulus after an 840 hour exposure to sulfuric acid. In the second investigation, prestressed tensile specimens were exposed to distilled water for 307 hours. The results showed that both strength and modulus were decreased by increasing prestress. A prestress of 50% of tensile strength resulted in a 12% decrease in strength and 14% decrease in modulus.