Utilizing in-situ measurements of ClO made in the same airmass more than once under different photochemical conditions during the Geophysica self-match flight during Euplex 2003, enabled us to examine our understanding of the kinetics of chlorine dimer cycle in detail. The kinetics of the chlorine dimer catalytic ozone loss cycle rely upon the thermal equilibrium between the monomer ClO and the dimer ClOOCl and the photolysis rate of the dimer ClOOCl. In this paper, we illustrate the kinetic constraints able to be determined by such a methodology. Our results suggest a thermal equilibrium constant a factor of 5 smaller and a ratio of photolysis rate to formation rate of ClOOCl a factor of 2 larger than the values based on the JPL recommendations.