Upper limits for the bimolecular rate constant for the reaction ClO+O3→ClOO+O2 have been measured directly between 233 and 413 K using a discharge-flow system at 1--2 torr total pressure. Production of ClO (which occurs quickly after thermal decomposition of ClOO in the presence of O3) was detected using Laser Magnetic Resonance spectroscopy, and distinguished from the reactant ClO through isotopic oxygen labelling. At 298 and 233 K, k1 was found to be less than 1.4¿10-17 cm3 molecule-1 s-1. At 413 K, k1=(4.0¿2.0)¿10-16 cm3 molecule-1 s-1. The experimental conditions were modelled using a kinetic simulation program to show that chemical and isotopic scrambling interferences were negligible. From these results, it is concluded that this bimolecular reaction does not contribute significantly to the observed ozone depletion within the antarctic vortex. ¿ American Geophysical Union 1990