Ring-opening polymerization of various lactones and lactides initiated by lanthanum isopropoxide has been investigated. Analysis of molecular weights and molecular-weight distributions of the resulting polymers shows that the ring-opening process is a controlled reaction, which is initiated by a variable number of isopropoxy groups. This number depends on both the monomer and the [monomer]/[initiator] ratio. Kinetic studies indicate that all the polymerizations are equilibrated, and the corresponding thermodynamic parameters and the equilibrium monomer concentrations have been calculated. Comprehensive kinetics carried out for ε-caprolactone abd δ-valerolactone polymerizations allows the determination of kinetic order relative to both monomer and initiator concentrations, along with that of the apparent rate constants of polymerization. It is also demonstrated that propagation occurs on aggregated (δ-valerolactone) or unaggregated (ε-caprolactone) active polymer chains and that the ring-opening process proceeds via [O-acyl] bond cleavages.