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Oxidative stress is the mainstream toxicological mechanism for the adverse health outcomes of ambient aerosols. However, our understanding of the crucial redox-active species affecting the oxidative potential of water-soluble aerosols (OP WS) remains limited. In this study, the OP WS of PM 2.5 in Beijing was measured using dithiothreitol (DTT) assay, including DTT consumption rate and ·OH formation rate. OP WS was more closely related to water-soluble organic compounds (WSOC) rather than transition metals. Laboratory simulations were conducted to investigate the effects of individual target species in the context of complex metal-organic interactions. The results showed that reducing WSOC can effectively decrease OP WS , while reducing Cu2+ increased OP WS. Parallel factor analysis demonstrated that OP WS was the most significantly correlated with the highly oxidized humic-like or quinone-like substances. Multiple linear regression showed that aromatic secondary organic carbon (SOC) (34.4%), other primary combustion sources of WSOC (20.0%), primary biomass burning WSOC (19.8%), transition metal ions (12.9%) and biomass burning SOC (12.8%) made significant contributions to DTT V. In addition to the anthropogenic sources of WSOC, the aged biogenic SOC also contributed to OH V , particularly in summer. Reducing anthropogenic WSOC was the key to the effective control of OP WS of PM 2.5 in Beijing. [Display omitted] • OP WS showed the strongest correlation with WSOC rather than transition metal ions. • Reducing anthropogenic WSOC can effectively decrease OP WS of PM 2.5 in Beijing. • OH V was greatly affected by the aged biogenic SOC in summer. [ABSTRACT FROM AUTHOR]