The photochemical behaviour of aniline-2,5-disulfonic acid (ADSA) in aqueous solutions upon excitation at 254 nm, 310 nm and 334 nm has been studied. A specific phototransformation into o-anilinesulfonic acid (o-AS) via a desulfonation process was observed. Such a reaction was efficiently inhibited by molecular oxygen. The quantum yield was found to be 0.46 and 9.0 x 10(-3) in deoxygenated and oxygenated solutions, respectively. This inhibition was shown to result from an energy transfer process to oxygen through the formation of the lowest triplet excited state as demonstrated by a nanosecond laser flash photolysis study. The bimolecular quenching rate constant of the triplet state by oxygen is found to be 2.5 x 10(9) mol(-1) L s(-1). A mechanism taking into account the experimental results is proposed for the photochemical desulfonation of aniline-2,5-disulfonic acid.