H-1 NMR spectra of mixtures of Bu4Sn and benzyl alcohol indicate the formation of a weak complex. Furthermore, Bu4Sn catalyzed polymerizations of epsilon-caprolactone at 100 degrees C an accelerated by addition of benzyl alcohol, but a coordination-insertion mechanism based on a reaction between two ligands of the Sn-catalyst was not definitely proven. In contrast it was found that Bu4Sn is sensitive to oxidation by oxygen (air), a process which yields mainly Bu2SnO and only a trace of Bu3SnOBu. Hence, the Bu2SnO dissolved in Bu4Sn is the main initiator, but added benyzl alcohol acts as coinitiator yielding benzyl ester chain ends. (C) 2000 Elsevier Science Ltd. All rights reserved.