The unimolecular decomposition of ethyltrimethylsilane, chemically activated by the insertion of a singlet methylene radical into a CH bond of tetramethylsilane, has been studied from 0.03 to 70 mm Hg in the presence of oxygen. The singlet methylene radicals were produced by diazomethane photolysis at 4358 Å. Using neopentane and trimethylsilane as internal standards, the specific rate constant for ethyltrimethylsilane decomposition was determined to be 4.5 × 104 sec-1. This rate correlates via RRKM theoretical calculations with thermal A factors in the range of 1014.8 to 1015.7 sec-1 for ethyl rupture and 1015.2 to 1016.4 sec-1 for methyl rupture from the Si atom of ethyltrimethylsilane. The range in these A factors reflects the uncertainties in the critical energies for these two paths.