Abstract: The impact strength of thermoplastic poly(ethylene terephthalate) molding compositions containing a reinforcing filler (preferably glass fiber), an epoxidized crystallization promoter (e.g., an epoxidized vegetable oil) and a nucleating agent (e.g., sodium stearate) is markedly improved by including in the composition a small amount of an organofunctional silane, i.e., a compound characterized by having a functional substituent at the end of a methylene chain bound to a silicon atom to which are also bonded a plurality of readily hydrolyzable groups. A few such compounds are gamma-aminopropyltriethoxysilane, gamma-glycidoxypropyltrimethoxysilane, gamma-aminopropyltriethoxysilane and gamma-glycidoxypropyltrimethoxysilane. The formulations may also contain a warp-reducing filler such as mica. Organofunctional silanes typically have application in thermoplastic polymers only as coupling agents, i.e.