Peroxy radicals are known for their role in tropospheric photochemistry as intermediates in the oxidation of volatile organic compounds, leading to the formation of ozone and organic nitrate compounds. Similarly, in the particle phase, peroxy radicals, considered a type of reactive organic species (ROS), are also involved in many chemical transformations and produce a consequential fraction of aerosols with an impact on health. Here, we show that peroxy radicals are efficiently produced at the air/water interface upon irradiation of an organic film made of a simple fatty acid (i.e., nonanoic acid). This source of peroxy radicals was quantified as 0.27 ppbv after interfacial titration of the peroxy radicals by nitric oxide in a photochemical flow reactor. Using a combination of proton transfer reaction-time of flight-mass spectrometry (PTR-ToF-MS) and ultrahigh performance liquid chromatography-heated electrospray ionizationhigh resolution orbitrap-mass spectrometry (UHPLC-HESI-HR Orbitrap-MS), the products of 2 this photochemistry were identified in the presence and absence of NO. The amount of peroxy 19 radicals produced by this photochemistry was comparable to those measured in surface water, or the ROS bounded to ambient secondary organic aerosols. Accordingly, the photochemistry of surfactant at the air/water interface might play a significant role in the health impact of organic aerosols. KEYWORDS Peroxy radicals, Reactive oxygen species, nonanoic acid, photochemistry, airwater interface. ASSOCIATED CONTENT Supporting Information. Determination of the peroxy radicals produced by the photochemical reaction of the nonanoic acid monolayer, Figure S1-S4 and Scheme S1. This material is available 300 free of charge via the Internet at http://pubs.acs.org.