This study investigates the kinetics of free radical polymerization of styrene and n-butyl acrylate carried out in the presence of N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide (DEPN). With this stable radical as chain growth moderator, it is demonstrated that the polymerization of these two monomers exhibits a controlled character. The mechanism of polymerization is essentially the same as that described for other "living"/controlled radical polymerizations: the chains form a large pool of dormant species that can be reversibly activated, and only a minute fraction of them propagate at a given time. Using dilatometry and electron spin resonance (ESR), the evolution of the concentration of polymeric radicals and that of DEPN could be measured as a function of time. It appears that these DEPN-mediated polymerizations are driven toward a pseudo-stationary state that is reached after an initial period of a few minutes. During this pseudostationary phase, the concentration of polymeric radicals and that of DEPN remained essentially constant, which allowed us to determine the (K) equilibrium constant between dormant and active species and also the rate constants of reversible activation (k(d)) and deactivation (k(rec)) for each monomer. For purposes of comparison, a series of polymerizations were simulated using the PREDICI package: both experimental and simulated data were found to fall in rather good agreement.