A mass-spectrometric study of the hydrogen-atom initiated decomposition of monosilane was made. Silyl radicals were generated by mercury photosensitization of hydrogen-monosilane mixtures which consisted of about 95 percent hydrogen. Under these conditions more than 94 percent of the Hg(/sup 3/P/sub 1/) atoms that are quenched by collision react with monosilane to form silyl radicals. The further reactions of the silyl radicals are being studied. (LK)