A series of S-alkyl thioformates, HC(O)SR, has been synthesized by formylation of alkane thiols and their behaviour upon electron impact has been investigated by using tandem mass spectrometry methodologies. When R > CH3, a prominent loss of a SH. radical is observed in the ion source as well as in the field-free regions. This reaction, which requires an important preliminary rearrangement of the molecular ion, gives hydroxycarbenium ions as evidenced by collisional-activation spectra and consecutive collisional-activation experiments. A mechanism involving ionized thietane intermediates is proposed. (C) 1997 by John Wiley & Sons, Ltd.