Abstract Mass spectra of various methoxymethyldisilanes were examined in detail. Characteristic features of the spectra of these compounds are:(a) the fission of the Si—C bond occurs more readily than that of the Si Si bond;(b) the methoxy group is eliminated as formaldehyde;(c) the silicon analog of an acyl cation is formed from the [M 15]+ ion;(d) the 1, 2-shift of the methoxy group is characteristic in the fragmentation process.