Exhaust emissions of NO, HNO2, and HNO3 in the near-field plume of two B747 jet airliners cruising in the upper troposphere were measured in situ using the research aircraft Falcon of the Deutsches Zentrum f¿r Luft- und Raumfahrt. In addition, CO2 was measured providing exhaust plume dilution rates for the species. The observations were used to estimate the initial OH mixing ratio OH0 and the initial NO2/NOx ratio (NO2/NOx)0 at the engine exit and the combustor exit by comparison with calculations using a plume chemistry box model. From the two different plume events, and using two different model simulation modes in each case, we inferred OH emission indices EI(OH)=0.32--0.39 g(kg fuel)-1 (OH0=9.0--14.4 ppmv) and (NO2/NOx)0=0.12--0.17. Furthermore, our results indicate that the chemistry of the exhaust species during the short period between the combustion chamber exit and the engine exit must be considered with respect to the amount of OH at the engine exit plane because OH is already consumed to a great extent in this engine section because of conversion to HNO2 and HNO3. For the engines discussed here the modeled OH concentration decreases by a factor of ~350 between combustor exit and engine exit, leading to OH concentrations of 1--2¿1012 molecules cm-3 (=0.3--0.7 ppmv) at the engine exit. ¿ 1998 American Geophysical Union