Mass spectrometry is used to evaluate the occurrence of thio-enol structures among the several possible tautomers of thiohydantoins and dithiohydantoins. Mass spectra of differently substituted thiohydantoins are examined looking for common mass spectral behaviors. Ion fragmentations from specific tautomers allow to predict the most stable thio-enol structure for both type of compounds. The mass spectrum of the alkylation product of 5,5- dimethyldithiohydantoin and the nuclear magnetic resonance spectra of the alkylation products of both 2- thiohydantoin and dithiohydantoin support the fact that the most likely thio-enolstructure is determined by the presence of one or two thio-carbonyl groups in the hydantoin molecule.