This study focuses on the effects of ozonation on the optical and photochemical properties of dissolved organic matter (DOM). Upon ozonation, a decrease in light absorption properties of DOM was observed concomitantly with a large increase in singlet oxygen (O-1(2)) and hydroxyl radical (OH) quantum yields (phi O-1(2) and phi(OH), respectively). The decrease in absorbance was linked to the reaction of DOM chromophores with ozone and OH, formed as a secondary oxidant, while the increase in phi O-1(2) and phi(OH) are linked to the formation of quinone-like moieties from the reaction of ozone with phenolic DOM moieties. Investigations using benzoic acid as a OH probe and methanol as a OH scavenger indicated that not only is OH formed, but that other hydroxylating species (OH-like) are also produced upon DOM photo-irradiation.