The concentrations of reactive oxygen species (ROS) in various particle size fractions were determined for the Taipei aerosols and for particles in vehicular exhausts. Sampling of particles was carried out at a sidewalk and an underpass by a cascade impactor. The sampling duration was 3 h during daytime at the sidewalk and 20-30 min during rush hours in the underpass. The collected particles were treated with a non-fluorescent probe dichlorofluorescin (DCFH2) that forms highly fluorescent dichlorofluorescin (DCF) when oxidized in the presence of ROS. Data on the fluorescence intensity of DCF were converted into equivalent hydrogen peroxide concentrations using calibration curves obtained from H2O2 assay. The equivalent H2O2 concentration was used as an indicator for the reactivity of ROS in particles. The concentration of ROS in ambient particles had good correlations with the intensity of photochemical reactions. The correlation coefficient between the ROS concentration and the O-3 concentration was higher for smaller particles. This indicates that the intensity of photochemical reactions is a major factor affecting ROS concentration in smaller particles, especially in ultrafine particles (aerodynamic diameter <0.18 mum). Moreover, for the same mass concentrations of particles, the content of ROS was found to be higher in smaller particles, except for ultrafine particles. This pattern was observed only in the particles sampled at the sidewalk, and not those in the underpass. The results suggest that, for the same particle mass concentration, smaller particles in ambient aerosols have higher ROS contents and, therefore, may have a greater potential to cause damage to the pulmonary cells.