A kinetic approach, recently developed to calculate the effect of exchange between a dormant and an active species in group transfer polymerization (GTP), has been applied to living free radical polymerization moderated by nitroxide stable free radicals. A general solution for the molecular weight distribution as a function of conversion has been derived. The solution depends only on the rate constants for propagation, the trapping of the growing chains by nitroxide radical, and the release of the growing chain. The general form of the solution is the same in GTP and the stable free radical-mediated polymerization (SFRP), except for the definition of the constants. Using the measured and known experimental rate constants for SFRP and fitting to the only unknown rate constant, that for reversible chain termination by the nitroxide radical, allow a quantitative prediction of the molecular weight distribution with conversion for bulk free radical living polymerization of polystyrene. In this way a good fit to the experimental polydispersity is obtained over a wide range of polymerization conditions. The calculated rate for the reversible termination is reasonable compared to known nitroxide-trapping reactions but is over 3 orders of magnitude slower than for a diffusion-controlled reaction, on the same order as the rate constant for polystyrene radical-radical termination. Nevertheless, because of the excess nitroxide present, trapping is fast enough to ensure a high rate of exchange of growing and dormant chains, resulting in narrow polydispersities at high conversion. The very good fit to this model indicates that neither initiation nor termination are important to the conversion dependence of the molecular weight distribution, as neither were taken into account in the kinetic model. This is further support for the current understanding of the mechanism and kinetics of the SFRP process. The polydispersity in the bulk living free radical polymerization mediated by nitroxide is controlled by the exchange rate between the growing and dormant chains. At high conversion where the rate of polymerization is high, there can be some irreversible chain termination and some autopolymerization.