The bulk polymerization of styrene at 125 degrees C in the presence of a PS-TEMPO adduct was studied with respect to the polymerization rate and the concentration of free TEMPO as a function of time, where PS is polystyrene and TEMPO is 2,2,6,6-tetramethylpiperidinyl-1-oxy. The results were perfectly consistent with the proposed kinetic scheme which assumes the existence of a stationary state with respect to both polymeric and nitroxyl radical concentrations and predicts that the polymerization rate of the nitroxide-mediated system is independent of the adduct concentration, being equal to the polymerization rate of the adduct-free system, i.e., the rate of thermal polymerization in the case of the styrene system studied here. The equilibrium constant K for the PS-TEMPO reversible reaction was estimated to be 2.1 x 10(-11) mol L(-1) on the basis of the dilatometric and electron spin resonance data. This value of K was indicated to be large enough to set the system under control. This work thus shows that in order for the "living" radical polymerization mediated by a stable nitroxyl radical (SNR) to proceed successfully, a constant supply of initiating radicals (by, e.g., thermal initiation) to make up for the loss of polymer radicals due to irreversible termination is essential as well as the frequent-reversible combination of polymeric and nitroxyl radicals. The total number of initiating radicals to be supplied in this way may be small compared with the number of polymer-SNR adducts so that they have no important influence on the molecular weight and its distribution of the product.