The thiomethyl anion (1) has been generated by fluorodesilylation of trimethylsilylmethanethiol in a variable-temperature flowing afterglow device. The proton affinity (1649 ± 12 kJ mol-1) and electron affinity (0.67 ± .13 eV) were determined and compared to a previously reported molecular orbital calculation. Isomerization via a 1,2-proton shift does not take place between -40° and 100°C despite a 156 kJ mol-1 driving force. Ion-molecule reactions of 1 were examined with a number of reagents including N20, O2, CS2, COS, and CO2, Hydride ion transfer was observed in every case, along with other products, and thermodynamic information has been derived. © 1990 American Society for Mass Spectrometry.