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$¿.nHNO3 and HSO4$¿.mH2SO4 clusters. Hydrated clusters of both ion families have also been detected. A reaction scheme is proposed to expalin 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 N2SO4 are briefly discussed.