A polymer stabilizer, p-butoxyphenylcyclohexyl-phosphinic acid, was introduced into an industrial polymer, a Bisphenol A-based polycarbonate, and the effects of this stabilizer on the mechanical and thermal properties were investigated. The mechanical properties were improved with small amount of stabilizers, while thermal properties were not effected significantly. For higher contents of stabilizer, both thermal and mechanical properties were deteriorated. This deterioration was caused by the increase of molecular distance between the chains, which lead to the reduction of polar forces and an increase in the free volume of the polymer. (C) 2001 John Wiley & Sons, Inc.