The pivotal role of the nitroxide concentration in bulk living polymerization of styrene was studied between 115 and 135 degrees C, using in situ electron spin resonance spectroscopy (ESR) to follow the concentration of the TEMPO stable free radical during the polymerization. Molecular weight and conversion were also followed on the same reaction mixtures using gel permeation chromatography and thermogravimetric analysis, respectively. While molecular weights were linear with conversion, to high conversion, there was an increase in the polymerization rate with time : nonideal behavior for a living polymerization. However, the TEMPO concentration also shows a slow decay as polymerization proceeds. Using the current mechanistic model, which predicts a polymerization rate inversely proportional to TEMPO concentration, this changing concentration was incorporated into the kinetic analysis. Except for low conversion in the lowest temperature polymerization, correction for the TEMPO concentration resulted in ideal, constant polymerization rate constants. While increasing the initial TEMPO concentration decreases the rate of polymerization dramatically, the corrected rate is independent of initial TEMPO concentration, again consistent with the current mechanism. From these corrected polymerization rates, the activation energy for the release of TEMPO from the growing chain end was estimated as 82 kJ/mol, considerably less than the previously observed value of 130 kJ/mol for the release of TEMPO from styrene 1-mers. Using TEMPO as a probe of irreversible chain termination, ESR shows that irreversible chain termination up to 75% conversion is limited to less than 2 chains in a hundred. It is concluded that the TEMPO-mediated polymerization is a living polymerization under the conditions of this study. To aid in the understanding of these living polymerizations that are based on reversible termination, a new term has been defined, the germination efficiency, which describes the yield of living chains in terms of the reversible terminating agent.