Composition measurements of negative ions in the stratosphere were performed at altitudes near 35 km by means of a balloon borne quadrupole mass spectrometer during two balloon flights. Some typical ion spectra are shown and discussed. The observed ions were identified as NO3 −.nHNO3 and HSO4 −.nHNO3.mH2SO4 clusters. Hydrated clusters of both ion families have also been detected. A reaction scheme is proposed to explain the data. The relative abundances of the ion mass peaks, as well as the measurement of minor constituents with large gas‐phase acidities, such as HNO3 and H2SO4 are briefly discussed.