Single crystals of 2-thiothymine (5-methyl-2-thiouracil) were c-irradiated at 77 K and studied using EPR spectroscopy. In addition to previously observed sulfur r radical, another sulfur-centered radical of p type was formed and analyzed at 100 K. The large values and the directions of the g-tensor principal elements undoubtedly confirmed the assignment. The observed isotropic 1:3:3:1 quartet structure of the EPR spectra, attributed to the couplings with methyl group protons, indicates the large delocalization of the spin along the 2-thiothymine molecule. Doublet splitting of the 1:3:3:1 quartets observed in some crystal orientations supports the idea that the pristine radical is an anion, formed by the capture of an electron at S(2) and subsequently neutralized by addition of a proton to S(2). From the crystal structure data it is concluded that the captured proton is the H 0 (3) of neighboring molecule, participating in the formation of the N 0 (3)-S(2) hydrogen bonding between two adjacent 2-thiothymine molecules in the unit cell.