Excited-state dynamics of 4-thiothymidine (S-4-TdR) and its photosensitization to molecular oxygen in solution with UVA irradiation were investigated. Absorption and emission spectra measurements revealed that UVA photolysis of S-4-TdR gives rise to a population of T-1(pi pi*), following S-2(pi pi*) -> S-1(n pi*) internal conversion. In transient absorption measurement, the 355 nm laser photolysis gave broad absorption (380-600 nm) bands of triplet S-4-TdR. The time-resolved thermal lensing (TRTL) signal of S-4-TdR containing the thermal component due to decay of triplet S-4-TdR was clearly observed by the 355 nm laser excitation. The quantum yield for S-1 -> T-1 intersystem crossing was estimated to be unity by a triplet quenching experiment with potassium iodide. In the presence of molecular oxygen, the photosensitization from triplet S-4-TdR gave rise to singlet oxygen O-2 ((1)Delta(g)) with a quantum yield of 0.50. Therapeutic implications of such singlet oxygen formation are discussed.