TiO2 and SiO2 surfaces modified with covalently bound organosilanes have been examined via two mass spectrometric surface analytical techniques: secondary ion mass spectrometry and electron-stimulated desorption. Hexamethyldisilazane and hexa(methyl-d3)disilazane vapors were reacted with the corresponding surfaces to yield monolayer or submonolayer coverages of bound trimethylsilyl and tri(methyl-d3)silyl groups. A 1.0-keV Ar ion bombardment of the modified TiO2 and SiO2 results in the ejection of secondary ions with the formulas [(CH3)nSi]+, where n = 3, 2, 1, and 0, in addition to ions originating from the oxide lattice. Various spectral features also indicate the formation of molecular ions by selvedge recombination and/or by molecular rearrangement during static SIMS analysis. A 1.0-keV electron bombardment of modified SiO2 yields the methylsilyl ion series observed in SIMS. Trimethylsilyl groups bound to TiO2 are ca. 5 times more stable under electron bombardment than the SiO2 system. Methylsilyl ions are produced, but no trimethylsilyl cation is detected in the case of the TiO2 substrate. The dependences of the parent ion signal with primary particle dose were examined, and, in light of these and related data, plausible damage mechanisms which lead to the decrease of the surface concentration of intact trimethylsilyl groups are discussed.