Mass spectra of selected thiomorpholides have been analysed and specific fragmentation assignments have been done to characterised and weigh co-existing keto and enol tautomers. The predictive value of this methodology in regard to keto-enol equilibria occurring in gas phase is not only supported by the influence of substitution nature and size of these but also by the good correlation found between the selected fragments abundances ratio, semi-empirical calcula- tion (AM1) of the corresponding heats of tautomerization and nuclear magnetic resonance data. The results show that the thioketo-thioenol equilibrium can be studied by mass spectrometry where ionization in the ion source has a negligible ef- fect on the position of that equilibrium and by GC/MS where solvent separation in the analytical column seems to pre- clude shifts from the equilibrium reached at the injection port of the gas chromatograph.Laboratorio de Estudio de Compuestos OrgánicosInstituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas