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Reactive oxygen species (ROS) are an important contributor to adverse health effects associated with ambient air pollution. Despite infiltration of ROS from outdoors, and possible indoor sources (eg, combustion), there are limited data available on indoor ROS. In this study, part of the second phase of Air Composition and Reactivity from Outdoor aNd Indoor Mixing campaign (ACRONIM‐2), we constructed and deployed an online, continuous, system to measure extracellular gas‐ and particle‐phase ROS during summer in an unoccupied residence in St. Louis, MO, USA. Over a period of one week, we observed that the non‐denuded outdoor ROS (representing particle‐phase ROS and some gas‐phase ROS) concentration ranged from 1 to 4 nmol/m3 (as H2O2). Outdoor concentrations were highest in the afternoon, coincident with peak photochemistry periods. The indoor concentrations of particle‐phase ROS were nearly equal to outdoor concentrations, regardless of window‐opening status or air exchange rates. The indoor/outdoor ratio of non‐denuded ROS (I/OROS) was significantly less than 1 with windows open and even lower with windows closed. Combined, these observations suggest that gas‐phase ROS are efficiently removed by interior building surfaces and that there may be an indoor source of particle‐phase ROS. [ABSTRACT FROM AUTHOR]