The kinetics of cationic polymerization is studied theoretically in accordance with a three-state mechanism which consists of two successive equilibria : the ionization/ion collapse equilibrium between covalent species and ion pairs, and the subsequent dissociation/association equilibrium between ion pairs and free ions. The number- and weight-average degrees of polymerization and the polydispersity index (PDI), <(P)over bar (w)>/<(P)over bar (n)>, are derived. The molecular weight distribution of the polymer generated from this mechanism is generally broader than that of polymers formed via a two-state mechanism, i.e. with only one equilibrium either between covalent species and ion pairs or between covalent species and free ions. Under this general mechanism, the exchange rate parameter, beta, is more complicated than that of two-state mechanisms and is a combination of the corresponding exchange rate parameters for these two mechanisms. The molecular weight distribution becomes narrower if dissociation is reduced by adding common counterions to the reaction system. Excess common ions lead to a two-state polymerization with covalent species and ion pairs only. On the other hand, if dissociation is very strong, the PDI is predominantly given by the rate of the ionization/ion collapse equilibrium unless the dissociation/association equilibrium is very much slower than the former one.