Reactive oxygen species (ROS) are believed to contribute to the adverse health effects of aerosols. We measured particle-bound ROS (PB-ROS) with an online instrument in two distinct environments, i.e., Beijing (China) and Bern (Switzerland). In both cities these exogenic ROS are predominantly related to secondary organic aerosol (SOA). PB-ROS content in SOA from various anthropogenic emission sources tested in the laboratory was comparable to that in the ambient measurements.