The catalytic photo-oxidation of 3,4-dihydroxybenzoic acid on TiO₂ has been studied by in situ ATR-FTIR in flowing water and in flowing wet air/nitrogen gas. In flowing water it was difficult to observe photodegradation intermediates despite photocatalytic action during UV illumination. In the flowing wet air/nitrogen system carboxylic acids and carbonates were observed. It was shown that water plays an important role in the formation of oxidation active species. Oxygen shows a prominent role for carboxylic acid degradation, but the photogenerated hole plays the important role for the 3,4-dihydroxybenzoic acid ring cleavage.