Peroxy radicals were measured by a PeRCA (Peroxy Radical Chemical Amplifier) instrument in the boundary layer during the DOMINO (Diel Oxidant Mechanisms In relation to Nitrogen Oxides) campaign at a coastal, forested site influenced by urban-industrial emissions in Southern Spain in late autumn. <br><br> Total peroxy radicals (RO<sub>2</sub>* = HO<sub>2</sub> + ΣRO<sub>2</sub>) generally showed a daylight maximum between 10 and 50 pptv at 13:00 UTC, with an average of 18 pptv over the 15 days of measurements. Emissions from the industrial area of Huelva often impacted the measurement site at night during the campaign. The processing of significant levels of anthropogenic organics leads to an intense nocturnal radical chemistry accompanied by formation of organic peroxy radicals at comparable levels to those of summer photochemical conditions with peak events up to 60–80 pptv. The RO<sub>2</sub> production initiated by reactions of NO<sub>3</sub> with organic trace gases was estimated to be significant but not sufficient to account for the concentrations of RO<sub>2</sub>* observed in air masses carrying high pollutant loading. The nocturnal production of peroxy radicals seems therefore to be dominated by ozonolysis of volatile organic compounds. <br><br> RO<sub>2</sub>* diurnal variations were consistent with other HO<sub>2</sub> measurements available at the site. HO<sub>2</sub>/RO<sub>2</sub>* ratios generally varied between 0.3 and 0.4 in all wind directions. Occasional HO<sub>2</sub>/RO<sub>2</sub>* ≥ 1 seemed to be associated with periods of high RO<sub>2</sub>* variability and with RO<sub>2</sub> interferences in the HO<sub>2</sub> measurement in air masses with high RO<sub>2</sub> load.