The gas-phase proton affinities of 2- and 4-thiouracil and 2,4-dithiouracil have been measured by means of Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. High-level ab initio calculations, in the framework of the G2(MP2) theory, have been carried out to establish the nature of the protonation site. Thiouracils behave as bases of rather similar moderate strength in the gas phase, the 2,4-dithiouracil being the most basic of the three. In all cases, the protonation takes place at the heteroatom attached to position 4, hence although, in general, thiocarbonyls are stronger bases than carbonyls in the gas phase, 2-thiouracil behaves as an oxygen base. For 2-thiouracyl and 2,4-dithiouracil, the most stable protonated conformer is the enol-enethiol form that cannot be formed by either direct protonation of the corresponding neutral or a unimolecular tautomerization of the oxygen or sulfur protonated species. We have shown that alternative mechanisms involving the formation of hydrogen bonded dimers between the protonated form and the neutral form, followed by appropriate proton transfers within the dimer, can be invoked to explain the formation of the most stable conformer.