Structure and coordinative association of the propagating species have been studied by H-1, Al-27, and C-13 NMR and viscosimetry in case of epsilon-caprolactone (E-CL) and lactides (LA) polymerization initiated by aluminum alkoxides. In toluene, aluminum triisopropoxide, a commonly used initiator, forms tetrameric (A(4)) and/or trimeric (A(3)) aggregates, which are dissociated upon the addition of the cyclic monomer. Actually, LA is responsible for the complete deaggregation of both A(3) and A(4) into single species (A(1)) trisolvated by the monomer (Al coordination number, CN = 6), whereas A(3) is selectively deaggregated by epsilon-CL. This behavior is consistent with the higher stability of A(4) compared to A(3). Whatever the monomer, three chains are initiated by the active A(1) species. When conversion of epsilon-CL is complete, the active aluminum trialkoxide sites are solvated by one (CN = 4) and more probably three (CN = 6) carbonyl groups of the attached poly-epsilon-caprolactone chains. At the complete LA conversion, only one carbonyl group of the polylactide chains is coordinated to Al (CN = 4). In THF, there is a competition between monomer and solvent for coordination to Al, which explains why polymerization is slower in THF than in toluene. In both toluene and THF, diethylaluminum monoalkoxide forms cyclic trimers that are solvated by the monomer (epsilon-CL) at least on the NMR time scale. Nevertheless, a fractional kinetic order in initiator is observed which gives credit to a fast reversible dissociation of the epsilon-CL-solvated trimers.