In this paper, a substantial set of simultaneous measurements of the sum of peroxy radicals, [HO[subscript 2] + RO[subscript 2]], NO[subscript 3], hydrocarbons (HCs), and ozone, taken at Mace Head on the Atlantic coast of Ireland in spring 1997, is presented. Conditions encountered during the experiment ranged from semipolluted air masses advected from Britain and continental Europe to clean air masses off the North and mid-Atlantic, where mixing ratios of pollution tracers approached Northern Hemispheric background mixing ratios. Average mixing ratios of peroxy radicals varied from 2.5 to 5.5 parts per trillion by volume (pptv) at night depending on wind sector, and were observed to decay only very slowly from late afternoon to early morning (0.1–0.5 pptv h[superscript −1]). Measurements of OH and HO[subscript 2] on two nights using the Fluorescence Assay by Gas Expansion (FAGE) technique give an upper limit for [OH] of 2.5×10[superscript 5] molecules cm[superscript −3] and an average upper limit [HO[subscript 2]]/[HO[subscript 2] + RO[subscript 2]] ratio of 0.27. A modeling study of the [superscript 1]/e lifetimes of the peroxy radicals, assuming no radical production at night, yielded mean lifetimes of between ∼8–23 min for HO[subscript 2] and 3–18 min for CH[subscript 3]O[subscript 2]. Given these lifetimes, it may be concluded that the peroxy-radical mixing ratios observed could not be maintained without substantial production at night. No significant correlation is observed between measured [HO[subscript 2] + RO[subscript 2]] and [NO[subscript 3]] under any conditions. Calculation of the reaction rates for ozone and NO[subscript 3] with hydrocarbons (HCs) shows that the ozone-initiated oxidation routes of HCs outweighed those of NO[subscript 3] in the NE, SE and NW wind sectors. In the SW sector, however, the two mechanisms operated at similar rates on average, and oxidation by NO[subscript 3] was the dominant route in the westerly sector. The oxidation of alkenes at night by ozone was greater by a factor of 4 than that by NO[subscript 3] over the whole data set. The HC degradation rates from the three “westerly” sectors, where tracer mixing ratios were relatively low, may be representative of the nighttime oxidative capacity of the marine boundary layer throughout the background Northern Hemisphere. Further calculations using literature values for OH yields and inferred RO[subscript 2] yields from the ozone-alkene reactions show that peroxy radicals derived from the ozone reactions were likely to make up the major part of the peroxy-radical signal at night (mean value 66%). However, the NO[subscript 3] source was of similar magnitude in the middle of the night, when [NO[subscript 3]] was generally at its maximum. The estimated total rates of formation of peroxy radicals are much higher under semipolluted conditions (mean 8.0×10[superscript 4] molecules cm[superscript −3] s[superscript −1] in the SE wind sector) than under cleaner conditions (mean 2.4×10[superscript 4] molecules cm[superscript −3] s[superscript −1] in the westerly wind sector). A model study using a campaign-tailored box model (CTBM) for semipolluted conditions shows that the major primary sources of OH, HO[subscript 2], and CH[subscript 3]O[subscript 2] (the most abundant organic peroxy radical) were the Criegee biradical intermediates formed in the reactions of ozone with alkenes.