Using a novel aircraft-borne automatic mass spectrometer, the odd-nitrogen gases NO, NO2, HNO2, and HNO3 were for the first time measured in a young exhaust-trail of a jetliner at cruise-altitude. The measurements, which took place at an altitude of 9.5 km and a distance of about 2 km from a DC-9 jetliner, revealed NO and NO2 to be present with abundances as large as 780 and 150 ppbv, respectively. The acids HNO2 and HNO3 reached abundances of 0.52 and 0.46 ppbV, which implies that only about 0.05% of the emitted reactive nitrogen experienced rapid conversion to the stable odd nitrogen reservoir nitric acid. Hence, most of the emitted odd-nitrogen remained in the reactive form NO and NO2, which affect ozone. Nitrous acid (HNO2) turns out to be an excellent tracer for young jet-aircraft plumes since it is short-lived and reaches only a very low atmospheric background abundance. The low HNO3-emission implies that HNO3-H2O nucleation and condensation in jet-aircraft plumes is hardly favored by the additional HNO3. However, it may still occur due to H2O emissions. ¿ American Geophysical Union 1992