An overview is given of the positive ion composition measurements performed with balloon-borne ion mass spectrometers in the Earth's atmosphere between 20 and 45 km. In particular, attention is paid to the derivation of the acetonitrile volume mixing ratio profile. The results are compared with model calculations, data obtained with airplane-borne instruments, and ground-level samplings. The consistency of the present picture is verified. The comparison with results of recent laboratory experiments show that electric field-induced collisional dissociation occurs in balloon-borne ion mass spectrometers. Probably, this is one of the major causes for the observed discrepancies between the data of airplane- and balloon-borne instruments.