The Cl2-sensitized continuous photolysis of O3 mixtures in O2 was investigated at 298 K using time resolved single wavelength and diode array spectroscopy to monitor the decay of O3 and the formation of OClO. &PHgr;-O3 was found to be 4.1¿0.4. This value, combined with the determination of the initial rates of OClO formation, allowed the evaluation of the following branching ratios into the three bimolecular disproportionation channels of ClO reactions; k3a/k3=0.40¿0.08, k3b/k3=0.42¿0.08, and k3c/k3=0.18¿0.02 where ClO+ClO→Cl2+O2 (3a), ClO+ClO→Cl+Cl+O2(3b) and ClO+ClO→Cl+OClO (3c). ¿ American Geophysical Union 1993