UARS observations of O3 and ClO (Microwave Limb Sounder), ClONO2 and HNO3 (Cryogenic Array Etalon Spectrometer), NO, NO2, and HCl (Halogen Occultation Experiment), and model calculations are used to produce an exposition of the different processes through which the reservoir gases ClONO2 and HCl are reformed at the end of the polar winter. Comparison of the observations within the polar vortices shows that HCl increases more rapidly in the Antarctic vortex in spring than in the Arctic vortex. Model analysis shows that this occurs because the O3 concentrations in the southern vortex fall well below those in the northern vortex. The Cl/ClO fraction calculated for the southern hemisphere is therefore up to 30 times higher, leading to rapid HCl formation by Cl+CH4. The concentrations of NO observed by HALOE are substantially lower for the northern hemisphere than for the southern hemisphere, even for similar values of the concentration of HNO3 and the production of NOx from HNO3 through photolysis and reaction with OH. This is consistent with the dependence of the NO/NOx ratio on the O3 concentration, i.e., the daytime production rate of NO2 via NO+O3 is reduced, leading to higher NO in the southern hemisphere. This higher concentration of NO also contributes to the rapid HCl increase as Cl production from ClO+NO is enhanced. ¿ American Geophysical Union 1995