PM2.5 filter samples were collected in summer 2005 at an urban background site in Augsburg, Germany. They were analyzed for polycyclic aromatic hydrocarbons (PAH) and their oxygenated derivatives (O-PAH) using gas chromatography/mass spectrometry. Oxygen free radical formation (reactive oxygen substances, ROS) was measured by electron spin resonance (ESR) spectroscopy after addition of spin trapping agent directly on the same filters. The concentrations of ambient, high-boiling PAH and O-PAH were highly correlated to ROS formation, even better than to particulate mass or number concentration. Correlations were most pronounced for some polycyclic aromatic monoketones (e.g., benz[de]anthracene-7-one), which are not yet reported in literature to be redox cycling active. The association found between ESR measurements and the presence of specific semivolatile organic compounds suggests an important influence of wood burning in PM2.5-associated ROS formation. These results indicate that further research on the relationship between radical formation and presence of specific O-PAH and semivolatile organic compounds (SVOC) are likely to provide a better understanding of the relationship between the source-dependent chemical composition of PM and the toxicological risks associated with PM exposure.