The activation-deactivation equilibrium of nitroxide-controlled radical polymerization of styrene at 123 degrees C was investigated. For this purpose the reaction solution was examined time dependently during the initial phase of the polymerization by using an SEC column combination providing a very good separation of the low-molecular weight species. By time-dependent measurement of the alkoxyamine concentration the activation rate of the alkoxyamines PhEt-TIPNO (N-tert-butyl-N-(2-methyl-1-phenyl-propyl)-O-(1-phenyl-ethyl)-hydroxylam ine) k(act) = 3.2 x 10(-3) s(-1) and PhEt-BIPNO (N-tert-butyl-N-(1-isopropyl-2-methyl-propyl)-O-(1-phenyl-ethyl)-hydroxy lamine) k(act) = 6.4 x 10(-3) s(-1) can be determined directly. Considering the Persistent Radical Effect theory, the measurement of the free nitroxide concentration allows to determine the pseudoequilibrium constant of dissociation/combination between dormant and active species for polystyryl-TIPNO and polystyryl-BIPNO, K = 7.5 x 10(-9) mol/L and 1.08 x 10(-8) mol/L, respectively. (c) 2007 Elsevier Ltd. All rights reserved.