The first measurements of ClOOCl in the stratosphere have been acquired from a NASA ER-2 aircraft, deployed from Kiruna, Sweden (68¿N, 21¿E), during the joint SOLVE/THESEO-2000 mission of the winter of 1999/2000. ClOOCl is detected by thermal dissociation into two ClO fragments that are measured by the well-known technique of chemical conversion, vacuum ultraviolet resonance fluorescence. Ambient ClO is detected simultaneously. Observations of the ratio /2 (estimated uncertainty of ¿25%, 1 σ) are used with a time-dependent photochemical model, to test the model representation of the ratios of kinetic parameters J/kProd and kLoss/kProd for day and nighttime observations, respectively. Here, kProd and kLoss are the rate constants for ClOOCl production and loss, respectively, and J is the photolysis rate of ClOOCl. The observations are in good agreement with J based upon the 2002 JPL recommended cross sections for ClOOCl <Sander et al., 2003>, if the true value of kProd is given by either the 2000 JPL recommendation <Sander et al., 2000> or the work of Trolier et al. <1990>. The larger values of kProd given by Bloss et al. <2001> and the 2002 JPL recommendation are consistent with the observations only if J is increased by a significant amount. This is accomplished if J is calculated with the larger ClOOCl cross sections measured by Burkholder et al. <1990>. The J values for ClOOCl based on the Huder and DeMore <1995> cross sections are too small, by factors of ~1.6 to 2.5 for all values of kProd, based on the observations. Nighttime results suggest that, for 190 < T < 200 K, the values for KEq (the equilibrium constant, equal to the ratio of kProd/kLoss) of Cox and Hayman <1988> and Avallone and Toohey <2001> are in best agreement with the observations.