Negative ion spectra obtained during four flights with a balloon-borne quadrupole mass spectrometer are reported and critically investigated. Ion abundances for NO3 - and HSO4 - core ions are reported and concentrations of HNO3 and H2SO4 at altitudes between 32 and 35 km are deduced. The detection of minor mass peaks in negative ion spectra obtained at an altitude of 32 km is discussed. Major mass peaks observed at lower altitudes (from 20 to 28 km) are mainly NO3 - due to core ions.