We have performed aircraft-borne measurements of peroxy radicals in the free troposphere over southern Germany (48 ¿N). The measurements were carried out on board a Dornier 228 research aircraft. At altitudes between 3.7 and 7.8 km measured peroxy radical concentrations were between 2¿108 and 7¿108 cm-3. Peroxy radicals were measured using amplifying chemical conversion of peroxy radicals to sulfuric acid via reactions with NO and SO2 and detection of sulfuric acid by ion molecule reaction-mass spectrometry (IMR-MS). The instrument was calibrated at the ground and thoroughly characterized by laboratory and in-flight diagnostic measurements. The accuracy of the present aircraft-borne measurements is estimated to be about ¿45%. Regular background measurements have been performed during the flights to determine the extent of spurious radical or H2SO4 production, which may occur through ion source produced radicals or thermal decomposition of PAN or HNO4 in the sampling system. ¿ 1998 American Geophysical Union