A study is made of the factors leading to the degradation in mechanical properties of glass reinforced polyester exposed to aggressive chemical environments such as H2SO4, HCl, and salt solutions at elevated temperatures. The penetration of water molecules, cations, and anions into the structure is described in terms of permeation rates as a function of time, temperature, pressure, glass content, resin type, and concentration of solutions. The permeation measurements are related to the diffusivity and solubility of the penetrating species in the structure and to the resultant degradation of mechanical properties.