The averages of the molecular weight distribution are derived for a "living" polymerization process which proceeds via active and "dormant" species and where the active species are formed by addition of a catalyst to a "dormant" species. Such a mechanism is applicable to group transfer and "living" cationic polymerizations (assuming that ion pairs are formed exclusively in the latter ease). Both equilibrium and nonequilibrium initial conditions are used for the calculation. The results are very similar to those obtained for degenerative transfer (i.e., direct exchange of activity between active and "dormant" species), The dominating parameter is beta = k(2)/(k(p)I(0)), where K-2 and k(p) are the rate constants of reversible deactivation and propagation, respectively, and lo is the initial initiator concentration (for degenerative transfer this parameter was given as beta = k(ex)/k(p), independent of initiator concentration). As an example, for beta > 1 the polydispersity index decreases with monomer conversion (after a marked increase at low conversions), coinciding with a common observation in group transfer and cationic polymerizations. In a limiting case, at full conversion, M(w)M(n) approximate to 1 + 1/beta. Differences between equilibrium and nonequilibrium initial conditions can only be seen for beta < 1. The results are compared to experimental data of the GTP of methyl methacrylate using nucleophilic catalysts and to the cationic polymerization of isobutylene. Typically, exchange is moderately fast (10 less than or equal to beta less than or equal to 100), thus the equilibrium initial conditions are sufficient for the calculations.