Chlorine catalytic chemistry, which destroys ozone while cycling chlorine between Cl, ClO, and Cl2O2, is the primary cause of the springtime Antarctic ozone hole. We have calculated the concentrations of Cl2O2 which are in equilibrium with midday ground-based, aircraft, and satellite observations of ClO in the Antarctic spring lower stratosphere. Two significant conclusions are presented here: (1) Using the JPL 94 recommended rates and photolysis cross sections, more than ~2.0 ppbv ClO in the polar lower stratosphere causes inferred total active chlorine to exceed the total chlorine budget. This limit is smaller than some reported ClO measurements. (2) Using smaller cross sections recently measured by Huder and DeMore [1995>, the amount of Cl2O2 in midday equilibrium with measured ClO is approximately doubled. Activated chlorine inferred from many measurements then exceeds total chlorine in the lower stratosphere, suggesting these cross sections may be too small. ¿ American Geophysical Union 1995