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