Abstract Direct evidence for formation of unstable methoxy cyclosiloxanes [(CH 3 O) 2 SiO] 2 (1) and [(CH 3 O) 2 SiO] 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 and ...