Dissolved organic matter (DOM) has recently been shown to reduce the transformation rate of various aqueous organic contaminants submitted to oxidation by excited triplet states, apparently by inhibiting the transformation of oxidation intermediates. The main goals of the present study were to evaluate in more detail the effect of concentration and type of DOM on the triplet-induced transformation rate of four selected organic compounds and to check for an analogous inhibition effect in the case of oxidation induced by hydroxyl radical. A marked inhibition by DOM of triplet-induced oxidation was observed for N,N-dimethylaniline (DMA) and the two antibiotics sulfamethoxazole (SMX) and trimethoprim (TRI), with DOM of terrestrial origin being a more effective inhibitor than DOM of aquatic origin. The results are important to understand the role of DOM both as a photosensitizer and as an inhibitor for the triplet-induced transformation of aquatic contaminants. In contrast, no DOM-induced reduction in second-order rate constant could be observed in competition kinetics experiments for the reaction of hydroxyl radical with a series of 15 organic compounds, covering several classes of aromatic contaminants, indicating that Suwannee River fulvic acid (SRFA) used as reference DOM does not affect this reaction mechanism.