Hydroxyl radical (OH) emissions are relevant for oxidation reactions in the post flame chemistry of exhaust gases emitted from jet engines. No direct measurements of OH concentrations are available to date due to the low abundance and the short lifetime of this radical species. The first application of a combined technique based on Raman scattering and laser-induced fluorescence (LIF) spectrometry is presented here for measurements in the exhaust gases of a commercial jet engine operated in a test rig. From the measurements, upper limits for OH concentrations in the exit plane were determined in the range of 90 ppbv for take off and 80 ppbv for ap-idle. The values are significantly below the predictions of model calculations based on HONO and HNO3 inflight concentration measurements presented recently. Possibilities for further increase of the detection sensitivity for OH are discussed. ¿ 1999 American Geophysical Union