The mechanisms of population of the lowest excited triplet states of, 2-thiothymine were investigated by means of CASPT2//CASSCF quantum-chemical calculations, with extensive atomic natural orbital basis sets of double-zeta quality (ANO-L-VDZP). Several key structures corresponding to equilibrium geometries, surface, crossings, minimum energy paths, and linear interpolation in internal coordinates were used to explain the ability to sensitize molecular oxygen. After population of the S-2(1)(pi pi*) state, the system evolves to the state minimum. At this point, and along the minimum energy path of the (1)(pi pi*) state, two main mechanisms related to the triplet and singlet manifolds can be visualized, leading the system to the lowest triplet state, T-1(3)(pi pi*).