Hydroxyl and hydroperoxy radicals are key species for the understanding of atmospheric oxidation processes. Their measurement is challenging due to their high reactivity, therefore very sensitive detection methods are needed. Within this study, the measurement of hydroperoxy radicals (HO 2 ) using chemical ionization combined with an high resolution time of flight mass spectrometer (Aerodyne Research Inc.) employing bromide as primary ion is presented. The 1 σ limit of detection of 4.5 × 10 7 molecules cm −3 for a 60 s measurement is below typical HO 2 concentrations found in the atmosphere. The detection sensitivity of the instrument is affected by the presence of water vapor. Therefore, a water vapor dependent calibration factor that decreases approximately by a factor of 2 if the water vapor mixing ratio increases from 0.1 to 1.0 % needs to be applied. An instrumental background most likely generated by the ion source that is equivalent to a HO 2 concentration of 1.5 ± 0.2 × 10 8 molecules cm −3 is subtracted to derive atmospheric HO 2 concentrations. This background can be determined by overflowing the inlet with zero air. Several experiments were performed in the atmospheric simulation chamber SAPHIR at the Forschungszentrum Jülich to test the instrument performance by comparison to the well-established laser-induced fluorescence (LIF) technique for measurements of HO 2 . A high linear correlation coefficient of R 2 = 0.87 is achieved. The slope of the linear regression of 1.07 demonstrates the good absolute agreement of both measurements. Chemical conditions during 15 experiments allowed testing the instrument’s behavior in the presence of atmospheric concentrations of H 2 O, NO x and O 3 . No significant interferences from these species were observed. All these facts are demonstrating a reliable measurement of HO 2 by the chemical ionization mass spectrometer presented.