The gas phase proton affinities of 5-methylhydantoin and its thio derivatives were theoretically studied through the use of high-level density functional theory calculations. The structure of all possible tautomers and their conformers were optimized at the B3LYP/6-311+(d,p) level of theory. Final energies were obtained at the B3LYP/6-311+(2df,2p) level. The imidazolidone derivatives 5-methyl-2,4-dioxo imidazolidine, 5-methyl-2-oxo-4-thio imidazolidine, 5-methyl-2-thio-4-oxo imidazolidine, and 5-methyl-2,4-dithio imidazolidine possess moderately strong proton affinities. Protonation at sulfur would be larger than protonation at oxygen. The most stable protonated forms of 2O4O and 2S4O have the proton attached to the heteroatom in position 2, whereas protonation of 2O4S and 2S4S preferentially takes place at position 4. The barriers for proton migration between the different tautomers are rather large. The energy decomposition analysis analysis of the OH+ and SH+ interactions suggests that the bonding interactions come mainly from the covalent bond formation. The contribution of the Coulomb attraction is rather small. © 2012 Wiley Periodicals, Inc.