The free-radical polymerization of styrene has been studied at 123 degreesC in the presence of a betaphosphonylated alkoxyamine (diethyl[(1,1-dimethyl-ethyl)(1-phenylethoxy)amino]-2,2-dimethylpropyl phosphonate) (styryl-DEPN). The persistent radical effect is undoubtedly observed experimentally until the effect of viscosity on the rate constant of termination is no longer negligible. The kinetic laws of the persistent radical effect allow us to calculate the pseudo equilibrium constant of dissociation/combination between dormant and active species K-polystyryl-DEPN = 6.1 x 10(-9) mol.L-1 at 123 degreesC. We have verified the reliability of this constant by computer simulation in comparison with the experimental data.