Observations indicate that extremely low concentrations of HCl prevail in the stratosphere just after the end of polar night in Antarctica. This is mostly explained by the fast heterogeneous reactions of HCl with ClONO2 and HOCl. Just prior to polar night normal gas phase partitioning indicates that HOCl and ClONO2 constitute only about 30% of the inorganic chlorine reservoir, the major portion being HCl. Thus about 60% of the initial HCl would remain (after titration with HOCl and ClONO2) by the end of polar night. We suggest that further processing may be effected through the reaction of N2O5 with HCl on polar stratospheric cloud surfaces, the N2O5 being produced from NO radicals formed by the action of galactic cosmic rays. The efficiency of this process is controlled by the relative rate of the competing heterogeneous reaction of N2O5 with H2O. Moreover, galactic cosmic rays form OH radicals that further augment HCl processing. Through model calculations we show that this mechanism can considerably enhance the conversion of HCl into active chlorine, leading to an almost 50% lower HCl concentration shortly after the end of polar night compared to cases where galactic cosmic rays were neglected. ¿ American Geophysical Union 1993