In-situ measurements of ClO made onboard the Geophysica aircraft on 30 January 2003 in the Arctic afford a novel approach to constrain the kinetic parameters governing polar stratospheric chlorine chemistry using atmospheric observations. The self-match flight pattern, i.e. sampling individual air masses twice at different zenith angles, was utilized by simulating the evolution of ClO mixing ratios between two 'matching' points using a photochemical model and optimizing the model parameters to fit the observations within a retrieval framework. Our results suggest a ClO/ClOOCl thermal equilibrium constant Keq a factor of 5 smaller and a ratio J/kf a factor of 2 larger than the values based on the JPL recommendations. This concurs with other studies based on observed ClOx partitioning and corroborates that our understanding of stratospheric chlorine chemistry is incomplete, particularly in the light of the most recent laboratory experiments pointing to a J/kf ratio almost an order of magnitude below the JPL recommendation. Copyright 2008 by the American Geophysical Union.