Excited-state dynamics of 6-azauracil (6-AU) and sensitized singlet oxygen formation in acetonitrile solution with UV irradiation were investigated for the first time. In the transient absorption measurement, the 248 nm laser photolysis gave a relatively intense absorption band at 320 nm (epsilon = 1100 +/- 100 dm(3) mol(-1) cm(-1)) and a broadband in the 500 - 700 nm region due to triplet 6-AU. The triplet 6-AU, decaying with the rate constant of (5.3 +/- 0.2) x 10(6) s(-1) in Ar saturated acetonitrile, was quenched by molecular oxygen with the rate constant of (2.5 +/- 0.1) x 10(6) dm(3) mol(-1) s(-1). The formation quantum yield of excited triplet 6-AU was estimated to be unity by acetone triplet sensitization and actinometry with benzophenone. The time-resolved thermal lensing signal of 6-AU was also observed by 248 nm laser excitation. In the presence of molecular oxygen, the sensitization from triplet 6-AU gave rise to formation of singlet oxygen O-2 ((1)Delta(g)) with a quantum yield of 0.63 +/- 0.03. Drastically different excited-state dynamics of aza-substituted uracil from normal uracil were clarified, and the mechanism for the enhancement of intersystem crossing by aza-substitution is discussed.