Observations of the ratio between the change in slant column abundance of OClO and that of ozone as a function of solar zenith are used to deduce the diurnal cycle of the daytime OClO column abundance above Antarctica during September 1987. This approach effectively normalizes other factors such as air mass factor changes and allows study of the photochemical variations of OClO during twilight. The data exhibits a marked increase in OClO at large solar zenith angles in the evening twilight (near 90¿--93¿), in agreement with model predictions. The increase is likely caused primarily by the attenuation of the OClO photolysis rate in the twilight. Knowledge of both the full diurnal and daily variations of OClO deduced from the data can be used to evaluate the destruction of the ozone column due to coupled chlorine-bromine chemistry. The inferred ozone loss above McMurdo Station assuming presently accepted photochemistry is 19.5¿10 Dobson units over the entire month of September, or about 20¿10% of the observed total column decline. This value is in general agreement with inferences drawn from in-situ measurements of ClO and BrO. In mid-September, available OClO and HOCl column measurements suggest that the chlorine-bromine and chlorine-hydrogen cycles respectively contribute about 22--28% and 3--4% of the observed rate of column ozone decrease at that time of the month. Model calculations constrained by ClO, BrO, and OClO measurements suggest that these mechanisms combined with photolysis of the ClO dimer can account for much, and possibly all, of the total ozone destruction rate observed in mid-September 1987. ¿ American Geophysical Union 1990