Complete trimethylsilylation of 20 (S)-protopanaxadiol (11), 20 (S)-protopanaxatriol (12) (Ginseng-sapogenins) and dammarenediol-II (13) was furnished by the action of trimethylsilylimidazole. The mass spectra of the pertrimethylsilyl ethers of 11, 12, and 13 (TMSi-11, -12, and -13) exhibited characteristic fragmentations around their C-20, which were diagnostic for structure determination of triterpenes of this type. The mass spectra of TMSi-ethers of the deuterated compounds of 11 (11-d₂ and -d₅) demonstrated that no 1, 2-elimination of TMSiOH at C-3 and -12 was involved in the fragmentation. With reference to the reports of the mass spectrometry of TMSi-oligosaccharides, the mass spectra of Ginseng saponins (ginsenosides-Rb₁ (1), -Rb₂ (2), -Rc (3), -Rd (4), -Re (5), -Rf (6), -Rg₁ (7), and -Rg₂ (8) were also studied, the results of which were found to be useful for identification and structure determination of oligoglycosides of this type.