The titanium dioxide mediated photocatalytic degradation of dimethyl phenylphosphonate (DMPP) has been investigated. DMPP can be completely decomposed by irradiation of an aqueous suspension of Ti02 in the presence of oxygen, though the total mineralization of organophosphonates takes longer than the complete decomposition of dimethyl phenylphosphonate. DMPP degrades faster at higher pH values and lower initial concentrations. The photocatalytic degradation reaction can be described by the first order kinetics. The product distribution, the effect of different substituents on the phenyl ring, the similarity of the initial products obtained from the Ti02 photocatalytic degradation and H202 photolysis indicate that the chemistry of photocatalytic degradation of dimethyl phenylphosphonate at the early stages is consistent with hydroxyl radical chemistry. A mechanistic study of the degradation of 180-labeled DMPP indicates that an addition-elimination mechanism is not likely for the demethylation step during the photocatalytic degradation process.