Abstract The mass spectrometric investigation of specifically deuterium and 13 C labelled 2- trimethylsilyl-l-phenoxyethanes proves that the dissociative ionization of β-silyl-substituted ethane derivatives (loss of PhO⊎; p-CH 3 C 6 H 4 O⊎; and C 4 H⊎ 9 from PhOCH 2 CH 2 SiMe 3, p-MeC 6 H 4 OCH 2 CH 2 SiMe 3 and CH 3 CH 2 CH (CH 3) CH 2-CH 2 SiMe 3, respectively) yields the non-classical bridge ethylene trimethylsilanium ion and not the ...