Stable radicals derived from inhibitor molecules were detected in the process of styrene polymerization. N-(1,4-dimethylpentyl)-4-nitroso-aniline and 2,4-dinitrophenol inhibitors were shown to produce nitroxyl radicals. Phenoxyl radicals come from 4-benzylidene-2,6-di-tert-butyl-cyclohexa-2,5-dienone. The radical structures were determined. The kinetics of radical formation was studied. These radicals can participate in the process of living radical polymerization and significantly affect the kinetics of polymerization. (C) 2003 Wiley Periodicals, Inc.