The polymerization of epsilon-caprolactone (ECL) initiated with aluminium triisopropoxide (Al((OPr)-Pr-i)(3)) trimer (A(3)) and/or tetramer (A(4)) was studied. The rate of A(3) reversible arrow A(4) interconversions in the diluted (less than or equal to 0,1 mol.L(-1)) C6D6, C6D6/epsilon CL, and THF/epsilon CL solutions were found to be slow, when compared with the rate of propagation. Comparison of the H-1 NMR spectra of the initiators with those of the polymerization mixtures revealed that A(3) is much more reactive than A(4) in their reactions with ECL. From the initiator reacted with ECL all three -(OPr)-Pr-i groups from Al((OPr)-Pr-i)(3) are transferred into the poly(epsilon CL) as end groups. Kinetic studies of polymerization confirmed the large reactivity difference between A(3) and A(4).