The mechanism of ring-opening polymerization of some lactones and lactide initiated by lanthanum isopropoxide has been comprehensively investigated. NMR and viscosity analyses demonstrated that three active polymer chains grow per lanthanum atom and that. depending on the coordinating ability of the monomer, propagation proceeds on aggregated or unaggregated active species. It is also demonstrated that inter- and intramolecular side reactions are limited and that lanthanum-based initiator selectivity in ring-opening polymerization is analogous to that of aluminum ones.