A detailed kinetic study of the thermolysis of sym-dimethoxytetramethyldisilane (I) has been carried out. This study provides strong support for our previous proposal that dimethyldimethoxysilane and dimethylsilylene are formed in an unimolecular and rate determining a-elimination step. Further, chemical evidence is reported which, for the first time, clearly and unambiguously demonstrates insertion of silylenes into the silicon-oxygen bond of alkoxysilanes. © 1969.