Marine aerosols produced by bursting bubbles at the ocean surface are highly enriched in organic matter (OM) relative to seawater. The importance of this OM in the photochemical evolution of marine aerosols, particularly as a source of reactive oxidants, is unknown but likely significant. To investigate oxidant production, nascent aerosols were generated by bubbling zero air through flowing Sargasso seawater and photochemical production of OH radical and hydroperoxide were quantified in aqueous aerosol extracts exposed to solar radiation. Extrapolation to ambient conditions indicates that OM photolysis was the primary in situ source for both OH (1.1 × 10-8 M s-1) and hydroperoxides (1.7 × 10-8 M s-1) in nascent aerosols; NO3- photolysis was the primary source in aged, acidified aerosols (1.4 × 10-7 and 4.1 × 10-8 M s-1, respectively). In situ OH photoproduction was comparable to gas-phase uptake whereas H2O2 photoproduction was slower. Results provide important constraiqs for poorly quantified oxidant sources in marine aerosols.