An analysis of data obtained during the second Airborne Arctic Stratospheric Expedition (AASE-II) was made with emphasis on aircraft exhaust plumes accidentally encountered during the mission. Twenty spikes were found with peak NOy increments ≥1 ppbv. The examination of CO and CO2 indicates that there was only one NOy spike having clearly corresponding spikes of both CO and CO2, and another four with unambiguous CO2 spikes. No significant increases were found for CH4 and N2O for these 5 spikes. The ratio of the excess CO2 and NOy compares well with the ratio of published subsonic aircraft emission indices. The study of the selected spikes from the DC-8 and another two spikes observed during other missions shows that the odd nitrogen other than NOx accounts for a very small percentage of the NOy increase associated with the observed spikes. ¿ American Geophysical Union 1994