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[Display omitted] • Effects of coexisting processes in Fe2+/HClO system were individually investigated. • Chlorinated and non-chlorinated DOM underwent different transformation pathways. • Anodic oxidation can destroy aromatics but not block the formation of organochlorine. • FeIVO2+ is insufcient to degrade Cl-containing moieties and hydrophilic components. • Chlorinated organic molecules persisted and inhibited DOM mineralization. Dissolved organic matter (DOM) transformations in complex electrochemical oxidation system, subjecting to simultaneous action of coexisting processes in the system and complexity of DOM, remain quite unexplored. In the present study, electrodialysis treated landfill leachate nanofiltration concentrate (LLNC-ED) was used to individually investigate DOM molecular transformations induced by three coexisting processes in electrochemical Fe2+/HClO system (anodic oxidation (AO), active chlorine (ACl), and electrochemical Fenton-type (EFT) processes) to unveil their competition and/or cooperation mechanisms on DOM transformation, using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and fluorescence excitation-emission matrix with parallel factor (EEM-PARAFAC). DOM transformations during AO underwent three steps: the addition of M(•OH) (heterogeneously formed •OH) in aromatics, the decomposition of fluorescent moieties and accumulation of aliphatic compounds, and the H abstraction in aliphatic compounds. In comparison, aromatic compounds more readily reacted with ACl species by electrophilic substitution and phenolic moiety oxidation after introducing the ACl process, followed by the formation of Cl-containing and quinone-type moieties, respectively. The EFT process could efficiently degrade non-chlorinated compounds but chlorinated organic molecules remained. Given that the chlorinated organic molecules were mainly formed by electrophilic aromatic substitution, preoxidation aimed at deconstructing aromatic moieties is proposed to prevent the generation of Cl-containing moieties to improve DOM mineralization efficiency. This study provides valuable reference for unveiling DOM transformation in electrochemical oxidation systems with various coexisting processes. [ABSTRACT FROM AUTHOR]