In the present study the kinetics of the cationic polymerization of lactones has been considered. The differences in basicities of a monomer and a polymer in various systems have been taken into account. The kinetic data on ε-caprolactone polymerization obtained have been discussed on the basis of the mechanism including the following reactions: 1) chain propagation; 2) reversible combination of the active centers by the carbonyl group of the monomer; and 3) reversible combination of the active centers by the carbonyl group of the polymer.