Excited state characteristics of aza analogues of nucleic acid bases, 8-azaadenine (8AA), 5-azacytosine (5AC), 8-azaguanine (8AG), and 6-azauracil (6AU), in acetonitrile solution were comprehensively investigated with steady state absorption and emission spectra, transient absorption measurements, emission measurements for the singlet oxygen molecule, and time-dependent density functional theory (TD-DFT) calculations. The triplet-triplet absorption spectrum of 8AA whose peak was 455 nm was observed for the first time. Sensitized singlet oxygen formation of 8AA was also observed in O-2-saturated acetonitrile with quantum yields of 0.15 +/- 0.02. It was concluded that there were two kinds of aza analogues of nucleic acid bases type A had substantial quantum yield for the intersystem crossing and potential of O-2 ((1)Delta(g)) formation (8AA and 6AU), and type B did not (5AC and 8AG). TD-DFT calculations indicated that type A molecules had a dark (1)n pi* state below the first allowed (1)pi pi* state, while both S-1 and S-2 states for type B molecules had a pi pi* character. It strongly suggested that the dark (1)n pi* state below the (1)pi pi* state would play an important role in the ISC process of aza analogues of nucleic acid bases.