The potential-energy surface of the lowest-lying triplet uracil tautomerization is comprised of the 13 possible tautomeric forms including uracil itself. Their structures are determined at the B3LYP/6-31 + G(d,p) level. Some characteristics of the tautomeric geometries are described. The intra-molecular proton transfer reaction mechanism and the variation of thermodynamic properties show that reaction enthalpy is the determining factor for the occurrences and populations of the stable tautomers. The relative stability ordering of all the triplet uracil tautomers is established. The proton affinities (PAs) and the deprotonation enthalpies (DPEs) of the atoms or bonds involved in the tautomerization process are calculated at both B3LYP and MP2 methods. The PA and DPEs values sensitively depend on the tautomeric form. The NI site has a greater PA value than other sites and the PA of 04 is greater than that at the N2 site in the same tautomer. The DPE of OH is larger than that of NH in the keto-enol tautomers and the DPE of O4H4 larger than that of O2H2. These results provide a rationale for the fact that the dioxo form of uracil is the most stable one among the tautomers. The relative energies of the uracil tautomers are then rationalized in terms of a second-order polynomial of the difference between their mean PAs and DPEs.