We describe a novel instrument for aircraft-borne measurements of peroxy radicals developed and built at the Max-Planck-Institut f¿r Kernphysik in Heidelberg. The instrument measures peroxy radicals using the chemical conversion of peroxy radicals to sulfuric acid and the detection of sulfuric acid by ion molecule reaction mass spectrometry. The peroxy radical measurement system was calibrated at the ground using the UV photolysis of H2O and was further characterized by laboratory and in-flight diagnostic measurements. Particular attention was paid to the effects of wall losses in the flow reactor and sampling system and the measurement efficiency for organic peroxy radicals. The conversion efficiency of methyl peroxy radicals to sulfuric acid was investigated by detailed laboratory studies and was found to be >80% for the conditions of our experiment. The instrument was employed for aircraft-borne measurements of peroxy radicals in the free troposphere over south Germany in combination with measurements of O3, NO, H2O, and J(O1D). Peroxy radical mixing ratios between 10 and 40 pptv were measured at altitudes between 3.8 and 7.8 km. A comparison of the measured peroxy radical concentrations with the results of a simple steady state estimate is presented. ¿ 1999 American Geophysical Union