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One technique used to measure concentrations of the hydroperoxy radical (HO₂) in the atmosphere involves chemically converting it to OH by addition of NO and subsequent detection of OH. However, some organic peroxy radicals (RO₂) can also be rapidly converted to HO₂ (and subsequently OH) in the presence of NO, interfering with measurements of ambient HO₂ radical concentrations. This interference must be characterized for each instrument to determine to what extent various RO₂ radicals interfere with measurements of HO₂ and to assess the impact of this interference on past measurements. The efficiency of RO₂-to-HO₂ conversion for the Indiana University laser-induced fluorescence–fluorescence assay by gas expansion (IU-FAGE) instrument was measured for a variety of RO₂ radicals. Known quantities of OH and HO₂ radicals were produced from the photolysis of water vapor at 184.9 nm, and RO₂ radicals were produced by the reaction of several volatile organic compounds (VOCs) with OH. The conversion efficiency of RO₂ radicals to HO₂ was measured when NO was added to the sampling cell for conditions employed during several previous field campaigns. For these conditions, approximately 80 % of alkene-derived RO₂ radicals and 20 % of alkane-derived RO₂ radicals were converted to HO₂. Based on these measurements, interferences from various RO₂ radicals contributed to approximately 35 % of the measured HO₂ signal during the Mexico City Metropolitan Area (MCMA) 2006 campaign (MCMA-2006), where the measured VOCs consisted of a mixture of saturated and unsaturated species. However, this interference can contribute more significantly to the measured HO₂ signal in forested environments dominated by unsaturated biogenic emissions such as isoprene.