Chlorine oxide (ClO) was measured in situ in the perturbed arctic vortex from a balloon launched near Kiruna, Sweden, 67¿N, at sunset on January 27, 2000. Between 20 and 23 km, the time-dependent behavior of ClO agrees with the functional form for the kinetics of ClO + ClO + M → Cl2O2 + M. The temporal change of ClO is described well by the integral of a rate expression that includes this reaction and thermal decomposition of Cl2O2. The value for the rate coefficient that best describes the observed decreases in ClO is consistent with the upper limit of the currently recommended value, and agrees well with the results of a recent laboratory study of ClO recombination at 190--195 K. These results reinforce the link between man-made chlorine compounds and ozone losses in the polar regions. ¿ 2001 American Geophysical Union