Dissolved organic matter (DOM) in the biotreated effluent of a municipal wastewater treatment plant was separated by XAD-8 and XAD-4 resins into four fractions: hydrophobic acids, non-acid hydrophobics, transphilics and hydrophilics. Ozonation with and without ultraviolet (UV) enhancement removed most UV-absorbing substances in the first 30 min achieving 78% and 63% reduction in UV254, respectively; the UV enhancement resulted in a greater reduction in dissolved organic carbon (DOC) (90% vs. 36%). Ozone reacted sequentially with aromatic hydrophobics, transphilics, and then hydrophilics; however, under UV, it reacted with all four organic fractions simultaneously. Low-MW hydrophilics were the most abundant fraction in the ozone-treated effluent.