evidence for formation of unstable methoxy cyclosiloxanes [(CH3O)2SiO]2 (1) and [(CH3O)2SiO]3 (2) in the gas phase has been obtained by a mass-spectral study of the vacuum pyrolysis of 3,3-dimethoxy-6-oxa-3-silabicyclo[3.1.0]hexane (4). Intermediates 1 and 2 have been detected in the mass spectra of the pyrolysis products of 4 as the corresponding molecular ions, and identified from their exact masses