Addition of glass fiber reinforcement significantly improved the chem. resistance and hydrolytic stability of thermoplastic resins. Eighteen resin composites were tested, and glass reinforced composites had a greater improvement in chemical resistance in non-aqueous environment. The percent strength losses of the composites were less than those of the unreinforced polymer. Composites closest to optimum theoretical reinforcement had the best chemical resistance. The relative resistance to long-term hydrolytic aging in 30% glass reinforced resins was polypropylene [9003-07-0] > polysulfone > modified I > PVC [9002-86-2] > nylon 610 [9011-52-3] > polyacetal > styrene-acrylonitrile copolymer [9003-54-7] > polycarbonate. Other thermoplastics tested were ABS [9003-56-9], polystyrene [9003-53-6], polyethylene [9002-88-4], nylon 6 [25038-54-4], nylon 66 [32131-17-2], polyurethane and polyester.