Persistent free radicals (PFRs) are emerging contaminants of increasing concern, yet their formation, fate, toxicity and health risk are poorly known. Thermal treatment, a common remediation technique to clean industrial soils, induces the formation of PFRs, which could paradoxically increase soil toxicity, contrary to the original objective of remediation. Actually, there is little knowledge on the formation and toxicity of PFRs in soils contaminated by polycyclic aromatic hydrocarbons (PAHs). Here we studied the generation of PFRs of soils spiked with anthracene and heated 1 h from 100 to 600 °C, using electron paramagnetic resonance. We also investigated the formation of reactive oxygen species (ROS), e.g. superoxide radical (O 2•− ), hydrogen peroxide (H 2 O 2 ) and hydroxyl radical (•OH), in the aqueous phase of thermal-treated soil, and the impact of heating on soil oxidative potential, wheat growth and green algae activity. Results showed that PFRs, ROS, soil oxidative potential, plant toxicity and algal toxicity show a similar trend with an increase from 100 to 300 °C, followed by a decrease to 600 °C. Scavenger trapping tests reveal that algal toxicity is mainly due to the generation of O 2•− , •OH and H 2 O 2 induced by anthracene-PFRs and that anthracene and anthracene-PFRs have negligible direct algal toxicity. Overall, our findings reveal the unintended formation of toxic compounds peaking at 300 °C during the thermal remediation of PAHcontaminated soils. These results should help to assess the environmental risk of thermally treated PAH-contaminated soils.