Ground-based Fourier transform infrared (FTIR) measurements, performed in the Arctic polar night using the Moon as an infrared light source, reveal already during the first half of December 1992 a considerable reduction in the amount of stratospheric HCl and ClONO2. Balloon sondes yield that in the altitude range 16-22 km an ozone reduction occurred as soon as the circulating air masses were irradiated by sunlight. Within the same altitude range several polar stratospheric cloud (PSC) events are being reported throughout the winter. Assuming that the reduction of HCl and ClONO2 had occurred in the same altitude range, where the ozone depletion and PSCs were observed, our measurements imply that almost all HCl and ClONO2 in that altitude range had disappeared. This means that the stratosphere was primed for ozone depletion very early in winter. The results confirm model studies that in the Arctic, already weak PSC events can lead to a strong conversion of chlorine reservoir compounds into active forms, initiating the ozone depletion, as soon as sunlight is available. ¿ American Geophysical Union 1995