Ozonation is a well-recognized process in advanced treatment of municipal secondary effluent for water reclamation. However, the transformation of dissolved effluent organic matter (dEfOM) during ozonation of real effluents, particularly at molecular level, has been scarcely reported. In this study, we performed ozonation treatments on real secondary effluents from two municipal wastewater treatment plants, and used Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and various spectroscopic techniques to probe the transformation of dEfOM at four ozone dosage levels (0.28, 0.61, 0.89, and 1.21 mg O3/mg DOC). Most of the precursors were unsaturated and reduced compounds (positive double bond equivalent minus oxygen per carbon ((DBE-O)/C) and negative nominal oxidation state of carbon (NOSC)), whereas the products were mainly the saturated and oxidized ones (negative (DBE-O)/C and positive NOSC). As the ozone dosage increased, the relative abundance of O8-19 species gradually increased in the ozonated samples, whereas an opposite trend was observed for O5-7S1 species. Further, we employed 18 types of reactions to represent the ozonation process, and found that the oxygenation reaction (+3O) possessed the largest number of possible precursor-product pairs, and CHON compounds possessed the highest reactivity. Besides the dominant oxygenation reactions, decyclopropyl (-C3H4) was relatively common reaction for CHON compounds, while it was oxidative desulfonation (-SH2) for CHOS ones. In addition, the transformation of precursors to products accompanied with the drop of (DBE-O)/C, and the increase of NOSC and the O/C ratio. The precursors with aromaticity and fluorescence were mainly correlated with the compounds featuring higher (DBE-O)/C and lower NOSC values. This study is believed to help better understand and improve the application of ozonation process in advanced treatment of real wastewater. Copyright © 2020 Elsevier Ltd. All rights reserved.