Ozonation of drinking and wastewater relies on ozone (O-3) and hydroxyl radical ((OH)-O-center dot) as oxidants. Both oxidants react with dissolved organic matter (DOM) and alter its composition, but the selectivity of the two oxidants and mechanisms of reactivity with DOM moieties are largely unknown. The reactions of O-3 and (OH)-O-center dot with two DOM isolates were studied by varying specific ozone doses (0.1-1.3 mg-O-3/mg-C) at pH( )7. Additionally, conditions that favor 0 3 (i.e., addition of an (OH)-O-center dot scavenger) or (OH)-O-center dot (i.e., pH 11) were investigated. Ozonation decreases aromaticity, apparent molecular weight, and electron donating capacity (EDC) of DOM with large changes observed when O-3 is the main oxidant (e.g., EDC decreases 63-77% for 1.3 mg-O-3/mg-C). Both O-3 and (OH)-O-center dot react with highly aromatic, reduced formulas detected using high-resolution mass spectrometry (O:C = 0.48 +/- 0.12; H:C = 1.06 +/- 0.23), while (OH)-O-center dot also oxidizes more saturated formulas (H:C = 1.64 +/- 0.26). Established reactions between model compounds and O-3 (e.g., addition of one to two oxygen atoms) or (OH)-O-center dot (e.g., addition of one oxygen atom and decarboxylation) are observed and produce highly oxidized DOM (O:C > 1.0). This study provides molecular-level evidence for the selectivity of O-3 as an oxidant within DOM.