The nature of negative ions in the altitude region 42-45 km has been investigated by means of a balloon borne mass spectrometer. Apart from the NO3 - and HSO4 - clusters, ions with different cores, which can be identified as CO3 -, HCO3 -, Cl- and ClO3 - were observed. The spectra have been used to estimate the sulfuric acid number density at 45.2 and 42.3 km altitude.