Organo-substituted 2,5-dihydro-1,2,5-oxoniastannaboratoles I (R = Et, R1 = nPr, CH2NMe2; R = iPr, R1 = Et) were prepd. by methanolysis of zwitterionic h2-(alkyn-1-ylborate)alkenyltin compds. Analogously, the bis[h2-(alkyn-1-yl-borate)alkenyltin] deriv. reacts with an excess of methanol to give a dimeric MeO-bridged 2,5-dihydro-1,2,5-oxoniastannaboratole II. Various intermediates could be identified by NMR spectroscopy, and the mol. structure of II was detd. by x-ray anal. The structures of several products of the methanolysis indicate that protolysis of an Sn-C: bond occurs first, followed by an MeO/alkenyl exchange reaction.