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 SiSi bond; (b) the methoxy group is eliminated as formaldehyde; (c) the silicon analog of an acyl cation is formed from the [M15]+ ion; (d) the 1,2-shift of the methoxy group is characteristic in