Observations of O3, HCl, NO, and NO2 from the Halogen Occultation Experiment (HALOE) provide a means to investigate chemical change in the lower stratosphere over Antarctica during the first 23 days of October 1992. Two long-lived species also observed by HALOE, HF and CH4, are used as tracers to identify a series of air parcels having similar Cly and NOy abundances. The set of parcels chosen using tracer analysis show uniformly low O3 mixing ratios, less than 1 ppm on the 480 K surface (about 18--20 km). HCl mixing ratio for those parcels with less than 1 ppm of ozone nearly tripled during the time period, and NO+NO2 abundances rose sharply, by a factor of 6 or 7. These trends in HCl, NO, and NO2 agree qualitatively with model calculations which show that the formation of HCl proceeds quickly when O3 levels fall so low that (1) the rate of the reaction Cl+O3→ClO+O2 slows and (2) the rate of the reaction NO+ClO→NO2+Cl becomes faster than the rate of the competing reaction NO+O3→NO2+O2. Under these conditions, Cl increases at the expense of ClO, and HCl is formed via the reaction Cl+CH4→HCl+CH3. Stratospheric chlorine is thus shifted from reactive species to the long-lived, reservoir molecule HCl. The repartitioning of the active chlorine family in favor of HCl halts the processes that destroy ozone and makes available active nitrogen in the form of NO and NO2. The investigation confirms earlier results and validates tracer analysis as a reliable method to probe chemical change in the lower stratosphere.¿ 1997 American Geophysical Union