The reaction of hydroxyl radical ((OH)-O-center dot) with the nucleic acid base analogue 2-thiouracil (1) has been studied by pulse radiolysis experiments and DFT. The generic intermediate radicals feasible for the (OH)-O-center dot reactions with 1, namely, one electron oxidation product (1(center dot+)), (OH)-O-center dot-adducts (3(center dot), 4(center dot), and 5(center dot)), and H-abstracted radicals (6(center dot) and 7(center dot)), were characterized by interpreting their electronic and structural properties along with calculated energetics and UV-vis spectra. Pulse radiolysis experiments showed that the transient formed in the reaction of (OH)-O-center dot with 1 in water at pH 6.5 has lambda(max) at 430 nm. A bimolecular rate constant, k(2) of 9.6 X 10(9) M-1 s(-1), is determined for this reaction via competition kinetics with 2-propanol. The experiments suggested that the transient species could be a dimer radical cation 2(center dot+), formed by the reaction of 1 with the radical cation 1(center dot+). For this reaction, an equilibrium constant of 4.7 X 10(3) M-1 was determined. The transient formed in the reaction of 1 with pulse radiolytically produced Br-2(center dot-) at pH 6.5 as well as Cl-2(center dot-) at pH 1 has also produced lambda(max) at 430 nm and suggested the formation of 2(center dot+). The calculated UV-vis spectra of the transient species (1(center dot+), 3(center dot), 4(center dot), 5 center dot, 6(center dot), and 7(center dot)) showed no resemblance to the experimental spectra, while that of 2(center dot+) (lambda(max) = 420 nm) agreed well with the experimental value and thus confirmed the formation of 2(center dot+). The 420 nm peak was due to sigma --> sigma* electronic excitation centered on a 2-center-3-electron (2c-3e) sulfur-sulfur bond [-S therefore S-]. 2(center dot+) is the first reported example of a dimer radical cation in a pyrimidine heterocyclic system. Further, 5-C and 6-C substituted (substituents are -F, -Cl, -NH2, -N(CH3)(2), -OCH3, -CF3, -CH3, -CH2CH3, n-propyl, phenyl, and benzyl) and 5,6-disubstituted 2-thiouracil systems have been characterized by DFT and found that the reaction (1 + 1(center dot+) --> 2(center dot+)) is exergonic (1.12-13.63 kcal/mol) for many of them.