We have derived chlorine monoxide (ClO) mixing ratio profiles within the Antarctic vortex on an hourly basis from ground-based measurements of pressure-broadened emission line spectra. This data set has provided the first opportunity for a detailed comparison between the output of a photochemical model and the measured in situ diurnal behavior of ClO in the Antarctic spring stratosphere. We stress the importance of the diurnal behavior in furnishing a short-term, crucial test of the catalytic chlorine chemistry which determines longer-term ozone depletion. We obtain excellent agreement between our measured and modeled diurnal change using the rate constants recommended in the 1994 Jet Propulsion Laboratory (JPL) evaluation, giving support to current understanding of perturbed chlorine chemistry in the Antarctic spring vortex. We have furthermore found that we can use our data to narrow the listed 1994 JPL uncertainty range for the ClO dimer formation rate constant and the equilibrium constant between ClO dimer formation and thermal dissociation. We show that the new limits we set on the dimer formation rate constant reduce the uncertainty in the daily rate of chlorine catalyzed ozone loss calculated from observed ClO concentrations by ~40% at 186--196 K. We find that a modeled total ozone loss rate including both chemistry and vertical transport, based on our measurements, agrees rather well with the amount and the linear trend of ozone loss seen throughout September in coincident balloon measurements. ¿ American Geophysical Union 1996