AbstractThe complete sorption‐desorption equilibrium isotherms determined at 23° C indicate that glass fiber‐reinforced polymer (GRP) sheets weathered outdoors for 3 years absorb larger amounts of water vapor than the non‐weathered sheets. Sorption and desorption Kinetic results at 23° and two levels of relative pressure (p/p0= 0.80 andp/p0= 0.975) show that the weathered sheets absorb water vapor at considerably higher rates than the non‐weathered sheets. Larger amounts of sorbed water, and faster sorption and desorption will result in respectively greater magnitudes and higher rates of dimensional changes (swelling and shrinking). This accelerates the overall effects of environmentally exerted stress‐fatigue on the GR