To investigate the mechanism of the controlled/"living＇＇ radical polymerization system, electron paramagnetic resonance (EPR) spectroscopy was applied to the atom transfer radical polymerization (ATRP) of styrene. Although only copper(I) species was added to the system initially, EPR signals of a copper(II) species were clearly observed in the polymerization of styrene initiated by l-phenylethyl bromide, l-phenylethyl chloride, and benzyl bromide in toluene solution or in bulk. As the polymerization proceeded, the concentration of copper(II) increased gradually until a steady state was reached. Correlation between the time dependence of the concentration of copper(II) species and the kinetics of the polymerization is also discussed for various ATRP systems. Our results indicate that 4-6% of the initial copper(I) species is converted to copper(II) species in polymerization of styrene at 110 degrees C. Investigation of the ATRP systems which contain an excess amount of copper(0) powder with a given amount of copper(II) species was also performed by EPR spectroscopy. Concentration of copper(II) species was progressively reduced by a reaction with copper(0), but the reaction was slow enough not to interfere with the control of radical polymerization.