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We investigated semiquantitative changes in almost 1000 dissolved organic matter (DOM) features during oxidation with 1 mg of O 3 per liter (mg O 3 /L), 4 mg O 3 /L, or 4 mg O 3 /L + 2.5 mg of H 2 O 2 per liter (advanced oxidation process, AOP) by unknown screening analysis with Orbitrap mass spectrometry. The consequential effects on formation of unknown disinfection by-products (DBPs) by chlorination were evaluated in laboratory-scale experiments. Several hundred unsaturated DOM features with positive oxygen-subtracted double bond equivalents per carbon ((DBE–O)/C) were decomposed by the ozone-only treatment and AOP. The AOP decomposed some saturated (negative (DBE–O)/C)) and reduced molecules, which had negative carbon oxidation states (C os). Several hundred saturated oxidation by-products were detected after ozonation and the AOP. After chlorination, the samples pre-treated with ozone alone resulted in higher formation of unknown DBPs than the AOP pre-treated sample or the sample without oxidation. Over half of the DBP precursors, estimated by electrophilic substitution, were not totally decomposed by any oxidation process, but they were increased after the ozone-only process and AOP. DBP precursors produced by the ozone-only process or AOP formed unique unknown DBPs. Therefore, post-treatment processes after oxidation and before chlorination are important to minimize formation of unknown DBPs. Image 1 • Changes of over 1000 DOM features by ozonation and peroxone were investigated. • Unsaturated DOM features were selectively decomposed by ozone and peroxone. • Several hundred of saturated oxidation byproducts were detected after ozonation and peroxone. • Ozone results in formation of unknown disinfection by-products more than peroxone. • Ozone and peroxone increase generate their specific unknown DBP precursors. [ABSTRACT FROM AUTHOR]