Al((OPr)-Pr-i)(3), which is one of the most efficient initiators for the living ring-opening polymerization (ROP) of lactones, lactides, and cyclic anhydrides, is known to be coordinatively associated in apolar solvents. In the present study, the effect of cyclic monomers on the coordinative structure of Al((OPr)-Pr-i)(3) in benzene was investigated by cryometry. gamma-Butyrolactone (gamma-BL), a nonpolymerizable lactone, was first studied as a model monomer. The cryometric Job diagram for the gamma-BL/Al((OPr)-Pr-i)(3) system in benzene established formation of a trisolvated six-coordinate [Al((OPr)-Pr-i)(3).3 gamma-BL] complex. This is in perfect agreement with the hexacoordinated Al atoms as reported by Al-27 NMR. The extension of the cryometric study to a polymerizable lactone, e.g., delta-valerolactone (delta-VL), confirmed that the single [Al((OPr)-Pr-i)(3).3M] species is the actual initiator for the ring-opening polymerization. Al-27 NMR shows that the same conclusion holds when other cyclic monomers, such as lactides and glutaric anhydride, are considered. The induction period of time that is usually observed in the ROP of lactones, lactides, and cyclic anhydrides may now be attributed to the dissociation of the Al((OPr)-Pr-i)(3) aggregates upon the addition of the cyclic monomer. Toluene solutions of living poly(epsilon-CL) and polylactide chains as initiated by Al((OPr)-Pr-i)(3) were also investigated by viscosimetry. The mean association degree ((DA)) over bar of the living chains was calculated from the intrinsic viscosity compared to the value measured for the parent hydrolyzed and purified polyester.