Photolysis of Cl2/Cl2O/O2 mixtures at 197 K has been used to place an upper limit of 4¿10-18 cm3 on the equilibrium constant for the ClO+O2↔ClO.O2 reaction at that temperature. This value is three orders of magnitude below the current NASA recommendation [DeMore et al., 1990>. Estimating S¿(ClO.O2) as 73 cal/mol-K, the bond energy for the complex is less than 7.7 kcal/mole. ¿American Geophysical Union 1990