2',3',5'-Tri-O-acetyl-6,8-dithioguanosine (taDTGuo) is a nucleoside derivative of drug 6-thioguanine and under further development as a potential photo chemotherapeutic agent due to its desirable properties of photosensitivity to UVA light and singlet molecular oxygen generation. The photochemical characteristics of taDTGuo under biological conditions (namely in aqueous solution) were intensively investigated by the steady-state absorption and emission, time-resolved near-infrared emission measurements, and quantum chemical calculations. taDTGuo was found to be held in sequential acid dissociation equilibria within pH 3.79-11.93. With the global fitting analysis of the absorption spectra at various pHs, two pK(a) values of the equilibria were determined to be 7.02 +/- 0.01 and 9.79 +/- 0.01. Quantum chemical calculations suggested that its mono- and dianionic species in the ground state should be 1-imide anionic form (N-1-taDTGuo(-)) and 1,7-di-imide anionic form (taDTGuo(2-)). taDTGuo generates a singlet molecular oxygen effectively and has pH-dependent quantum yields. In conclusion, taDTGuo would be very useful as a potent agent for photochemotherapy under certain carcinomatous pH conditions.