Background: The oxidative potential (OP) of particulate matter (PM) has been proposed as a more health relevant metric than PM mass. Different assays exist for measuring OP and little is known about how the different assays compare. Aim: To assess the OP of PM collected at different site types and to evaluate differences between locations, size fractions and correlation with PM mass and PM composition for different measurement methods for OP. Methods: PM and PM was sampled at 5 sites: an underground station, a farm, 2 traffic sites and an urban background site. Three a-cellular assays; dithiothreitol (OP), electron spin resonance (OP) and ascorbate depletion (OP) were used to characterize the OP of PM. Results: The highest OP was observed at the underground, where OP of PM was 30 (OP) to >600 (OP) times higher compared to the urban background when expressed as OP/m and 2-40 times when expressed as OP/μg. For the outdoor sites, samples from the farm showed significantly lower OP and OP, whereas samples from the continuous traffic site showed the highest OP for all assays. Contrasts in OP between sites were generally larger than for PM mass and were lower for OP compared to OP and OP. Furthermore, OP/μg was significantly higher in PM compared to PM, whereas the reverse was the case for OP. OP and OP were highly correlated with traffic-related PM components (i.e. EC, Fe, Cu, PAHs), whereas OP showed the highest correlation with PM mass and OC. Conclusions: Contrasts in OP between sites, differences in size fractions and correlation with PM composition depended on the specific OP assay used, with OP and OP showing the most similar results. This suggests that either OP or OP and OP can complement each other in providing information regarding the oxidative properties of PM, which can subsequently be used to study its health effects.