Polymerization of epsilon-caprolactone (CL) initiated with tin(II) octoate (tin(II) 2-ethylhexanoate, (Sn(Oct)(2))) in the presence of butyl alcohol (BuOH) or water and conducted in tetrahydrofuran (THF) as a solvent at 80 degrees C was studied using MALDI-TOF mass spectrometry. Formation of the following populations of macromolecules was revealed: Bu[O(O)C(CH2)(5)](n),OSnOct, Bu[O(O)C(CH2)(5)](n)Oct, Bu[O(O)C(CH2)(5)](n)OH, H[O(O)C(CH2)(5)](n)Oct, H[O(O)C(CH2)(5)](n)OH, macrocyclics [O(O)C(CH2)(5)](n), and macrocyclics with incorporated tin(II) alkoxide moieties [O(O)C(CH2)(5)](n)OSn. Thus, the most rewarding has been a direct observation of species with a tin atom covalently bonded with the polyester chain, at least for two populations of macromolecules (i.e., Bu[O(O)C(CH2)(5)](n)OSnOct and [O(O)C(CH2)(5)](n)OSn cyclics). Identification of the tin-containing macromolecules was based not only on the agreement between the observed m/z and the calculated molar mass values but also on the particular isotopic distribution provided by the tin atom. This result is in favor of the mechanism postulating propagation on the tin(II) alkoxide as the active center.