Ion spectra obtained during three balloon flights performed in June 1982, September 1982, and September 1983 are reported. The lower altitude limit of the measurements is imposed by the decrease of the instrument sensitivity and the upper limit by present ballooning techniques. At high altitudes the ion spectra are disturbed by contamination, whereas at low altitudes, ion cluster breakup is a severe problem. Nevertheless knowledge of relative abundances of the different negative ion mass peaks, mainly belonging to the NO3-(HNO3)n and HSO4-(HNO3)m(H2SO4)t cluster families allows the derivation of concentrations of H2SO4 in the altitude region of 22--45 km. At 45 km an increased SO2 mixing ratio, possibly due to volcanic activities, was estimated from the negative ion mass spectra. The data are discussed in terms of previous measurements and models. Special interest is given to the sulfur chemistry and the implications on aerosol formation.