One technique used to measure concentrations of the hydroperoxy radical (HO<sub>2</sub>) in the atmosphere involves chemically converting it to OH by addition of NO and subsequent detection of OH. However, some organic peroxy radicals (RO<sub>2</sub>) can also be rapidly converted to HO<sub>2</sub> (and subsequently OH) in the presence of NO, interfering with measurements of ambient HO<sub>2</sub> radical concentrations. This interference must be characterized for each instrument to determine to what extent various RO<sub>2</sub> radicals interfere with measurements of HO<sub>2</sub> and to assess the impact of this interference on past measurements. The efficiency of RO<sub>2</sub> to HO<sub>2</sub> conversion for the Indiana University Laser-Induced Fluorescence – Fluorescence Assay by Gas Expansion (IU-FAGE) instrument was measured for a variety of RO<sub>2</sub> radicals. Known quantities of OH and HO<sub>2</sub> radicals were produced from the photolysis of water vapor at 184.9 nm, and RO<sub>2</sub> radicals were produced by the reaction of several volatile organic compounds with OH. The conversion efficiency of RO<sub>2</sub> radicals to HO<sub>2</sub> 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 RO<sub>2</sub> radicals and 20 % of alkane derived RO<sub>2</sub> radicals were converted to HO<sub>2</sub>. Based on these measurements, interferences from various RO<sub>2</sub> radicals contributed to approximately 35 % of the measured HO<sub>2</sub> signal during the Mexico City Metropolitan Area (MCMA) 2006 campaign, where the measured VOCs consisted of a mixture of saturated and unsaturated species. However, this interference can contribute more significantly to the measured HO<sub>2</sub> signal in forested environments dominated by unsaturated biogenic emissions such as isoprene.