Abstract Titanium dioxide nanoparticles (nTiO2) form reactive oxygen species (ROS) under irradiation by ultraviolet light (UV). This known photocatalytic activity may finally affect the presence and toxicity of organic environmental chemicals, which have not yet been studied at ambient UV intensity. The authors used a three-factorial design to evaluate the interaction of the carbamate insecticide pirimicarb (initial nominal concentration, 20 µg/L), ambient UV irradiation (40 W/m2 for 15 min), and nTiO2 (∼100 nm; 2.0 mg/L). Pirimicarb, pirimicarb × UV, and pirimicarb × nTiO2 treatments revealed a median immobilization of Daphnia magna after 72 h ranging between 70 and 80%. This effect seemed to be caused by the initial nominal pirimicarb concentration. However, UV irradiation before an exposure of daphnids in the presence of 2.0 mg nTiO2/L reduced pirimicarb concentrations to values below the limit of quantification, likely because of the formation of ROS. This reduction was associated with an almost complete removal of toxicity for D. magna. Furthermore, during a second experiment, 0.2 mg nTiO2/L in combination with 15 min UV irradiation reduced pirimicarb concentrations by approximately 30%. These results indicate a detoxification and therefore remediation potential of the combined application of nTiO2 and UV irradiation at ambient levels. This potential has not been documented to date in surface waters, where nTiO2 concentrations in the low to medium µg/L range may occur. Environ. Toxicol. Chem. 2012;31:518–523. © 2011 SETAC