Significant abundances of chlorine oxide (ClO) were observed throughout the lowermost stratosphere at high latitudes during winter from the NASA DC-8 aircraft during the SAGE III—Ozone Loss and Validation Experiment and Third European Stratospheric Experiment on Ozone 2000 (SOLVE/THESEO 2000) campaign. Mixing ratios of ClO averaging 15–20 parts per trillion by volume (pptv) were observed near the tropopause, a region where ClO abundances are usually only a pptv or less at lower latitudes. The ratio of ClO to inferred inorganic chlorine ([ClO]/[Cly]) was found to be largest (∼7%) in air characterized by low abundances of ozone (∼100–250 parts per billion by volume (ppbv)). This was the region where cirrus clouds were also observed occasionally during the measurement period, although abundances of ClO directly within cirrus clouds were not significantly different than background abundances. Nonzero instrument signals during darkness are attributed to detection of ∼5–15 pptv of OClO. BrO mixing ratios between 2 and 4 pptv are sufficient to produce these amounts of OClO, assuming daytime mixing ratios of ClO between 15 and 20 pptv. At these levels of ClO and BrO, approximately 10% of the ozone at these altitudes is chemically destroyed per month in springtime by reactions of ClO and BrO, representing an effective loss process for ozone near the high-latitude tropopause.