Hydroxyl radical (OH) emissions are relevant for oxidn. reactions in the post flame chem. of exhaust gases emitted from jet engines. No direct measurements of OH concns. 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 com. jet engine operated in a test rig. From the measurements, upper limits for OH concns. in the exit plane were detd. in the range of 90 ppbv for take off and 80 ppbv for ap-idle. The values are significantly below the predictions of model calcns. based on HONO and HNO3 inflight concn. measurements presented recently. Possibilities for further increase of the detection sensitivity for OH are discussed.