Gaseous negative ions were mass spectrometrically measured in the exhaust plume of a jet aircraft in flight. Using a quadrupole mass spectrometer operated in a high-pass mode, it was found that by far most of the ions had mass numbers >450 amu (atomic mass units) and number densities which markedly exceeded the number densities of ambient atmospheric ions. The latter were observed outside the exhaust plume and had mostly mass numbers <200 amu. Both their large numbers and large concentrations strongly suggest that the massive ions observed inside the plume are chemiions which were produced by the jet engines. The low fuel sulfur content (22 &mgr;g/g) suggests that the massive ions consist at least partly of species other than sulfuric acid. By interaction with exhaust gases these chemiions experienced rapid chemical transformation and growth in the early exhaust plume already at plume ages <0.4 s. ¿ 1999 American Geophysical Union