Monsanto and Owens Corning Fiberglas Europe have evaluated a new class of polyphenyl based polymer processing aids for glass reinforced thermoplastics and alloys. Liquid partly hydrogenated polyphenyls and solid polyphenyl waxes have been evaluated in glass reinforced formulations based on acrylonitrile-butadiene-styrene copolymers, rubber modified styrene-maleic anhydride copolymers, polypropylene, polycarbonate/abs and polyphenylene oxide/high impact polystyrene alloys. I all cases, the addition of polyphenyls improves the melt flow of injection moulding formulations without affecting the mechanical characteristics of the products.Generally, an improvement in flexural and tensile properties is observed when polyphenyls are added. In rubber modified styrene-maleic anhydride copolymers, the addition of polyphenyls results in a definite increase in notched izod impact. In general, the heat distortion temperature of the products is not very affected and we observe an improvement in surface appearance of the moulded parts.Finally, due to the chemical inertness typical of polyphenyls, the products modified with these processing aids have demonstrated a better hydrolytic stability than products containing ester or amide based processing aids or lubricants.