This work focuses on the mechanism of the pseudoliving free radical bulk polymerization of styrene, which produces narrow polydispersity resin, using nitroxide radicals to reversibly terminate the growing polymer chain. The polymerization kinetics and the rate of breaking of the nitroxide to polymer NO-C bond were studied. The polymerization gives an increase in molecular weight that is linear with conversion to high conversion, gives polydispersities that are narrow (<1.3) and relatively insensitive to conversion, and shows first-order kinetics in monomer concentration. Calculations demonstrate that the kinetics of polymerization greatly reduce the termination of the polymerization by the reaction of two growing chains, which is the predominant mechanism in conventional free radical polymerization. It is concluded that this nitroxide-mediated free radical polymerization is a living polymerization. To study the stability of the NO-C bond, model compounds, comprising a styrene unit terminated at either end with a benzoyloxy moiety and a nitroxide moiety, were prepared and characterized. These compounds were heated in situ in the ESR cavity to generate nitroxide radicals. Radical concentrations were followed as a function of time and temperature, and an activation energy for cleavage of the NO-C bond was calculated. A proposed mechanism fits the available kinetic data well, and measured activation energies for nitroxide compounds correlate with rates of polymerization.