In situ flight measurements of the hydroxyl (OH) and hydroperoxyl (HO2) radicals were performed in the exhaust plume of a Boeing 757 and in the contrail of a Douglas DC-8 flying under typical cruise conditions in the upper troposphere and lower stratosphere during the SUCCESS mission of April and May 1996. Measurements were made in a range of exhaust plume ages from roughly 10 seconds to 20 minutes. OH mixing ratios were found to be greatly enhanced in the plumes, to 2--5 pptv from a background of 0.3--0.5 pptv in young plumes, and to 0.6 pptv from a background of ≤0.2 pptv in older contrails. HO2 mixing ratios were heavily suppressed in the plumes, to 2--4 pptv from a background of 10--20 pptv, for both young plumes and older contrails. ¿ 1998 American Geophysical Union