To obtain more insight in the ring-opening polymerization of lactones, complex formation of the initiators tetraphenyltin, stannous octoate, tin tetrachloride, aluminium bromide and triisobutylaluminium (TIBA), and the monomers L(−)-lactide, D,L-lactide and glycolide was studied by i.r.-spectroscopy. When equimolar benzene or toluene solutions of initiators and monomers were combined, only complexes of aluminium bromide and D,L-lactide or glycolide, and of TIBA and D,L-lactide or glycolide were observed. The complex formation was studied in detail by varying the initiator and monomer concentrations. From these results and theoretical considerations, it is concluded that complexes are formed by the coordination of a carbonyl oxygen of monomer and the aluminium atom of initiator. The corresponding polymers were formed when TIBA was used as an initiator. When AlBr, was used, polymers were obtained only when traces of water were added. It is concluded that in the case of AlBr3 the actual initiating species is HBr and that the polymerization initiated with TIBA proceeds very probably through a coordinated insertion of the lactone monomer into the aluminium-carbon bond.