The photochemistry of anthraquinone-2-sulfonate (AQ2S) was studied as a function of pH, combining laser flash photolysis and steady-state irradiation experiments, with the additional help of a computational study of energy levels. Two out of the three transient species produced upon irradiation of AQ2S can be involved into the degradation of dissolved molecules, and also AQ2S in its ground state is degraded. The reactive transients are less stable but often more reactive under acidic conditions, which modulates the pH trend of the photodegradation of the adopted organic substrates (furfuryl alcohol, benzene, nitrobenzene). The ability of the excited states of irradiated AQ2S to simulate the reactivity of singlet oxygen upon degradation of furfuryl alcohol, and that of the hydroxyl radical by producing phenol from benzene, can have important consequences. Furfuryl alcohol and benzene are widely adopted probe molecules for the respective quanti. cation of singlet oxygen and the hydroxyl radical in many systems, among which are natural waters under irradiation. This study shows that the interference of AQ2S on singlet oxygen determination would be higher in acidic or basic than in similar to neutral conditions, while in the case of the hydroxyl radical the interference would increase with pH. Processes analogous to those studied could account for the interference of coloured dissolved organic matter on the quantification of singlet oxygen, observed in previous studies.