Three composition measurements of stratospheric positive ions conducted during daytime at 33-37 km altitutde by use of balloon-borne mass spectrometers revealed the presence of two families of ion species, proton hydrates (PH) and non-proton hydrates (NPH) having about equal abundances. The major PH are H+(H2O)3, H+(H2O)4 and H+(H2O)5 among which the fourth hydrate is by far most abundant. Among the NPH which possibly have the form H+x1(H2O)m, with x having mass 41¿1,96¿1, (H+x(H2O)3) is most abundant. Arguments are put forward inferring a formation of NPH from PH reacting with a stratospheric trace gas x whose proton affinity exceeds that of water and whose concentration is estimated to be 7⋅104 cm-3 at 37 km altitude. The possibility of probing atmospheric trace gases by in situ ion composition measurements is discussed and upper abundance limits are given for various gases.