Recent in situ measurements with balloon borne quadrupole mass spectrometers, between 20 and 45 km altitude, are reviewed and discussed. The major stratospheric positive ions observed are proton hydrates [H+(H2O)n] and non proton hydrates of the form H+Xm(H2O)2. The data analysis allows a derivation of the vertical mixing ratio profile of X (most probably CH3CN), which is compared with recent model calculations. From negative ion composition data, showing the presence of NO3- and HSO4- cluster ions, the density of sulfuric acid in the stratosphere is deduced. The implications of these findings on our understanding of the sulfur chemistry is briefly treated. Finally some other aspects such as contamination, cluster break up and the use of stratospheric ion mass spectra for determination of thermochemical data and other minor constituents are discussed.