Measurements of OClO total column amounts during the cold Arctic winter 1999/2000 retrieved from observations by the satellite instrument Global Ozone Monitoring Experiment (GOME) are presented. OClO is formed as a minor product of the reaction of BrO + ClO and thus serves as an indicator for a stratospheric chlorine activation. As a result of the good spatial and temporal coverage of GOME, it is possible to follow the temporal development of the stratospheric chlorine activation during the winter and spring on a daily basis. An initial weak chlorine activation was observed during mid November, shortly after stratospheric temperatures were sufficiently low that formation of polar stratospheric clouds resulted. Strong chlorine activation started around 22 December, when PSC formation was possible over a large altitude range. Chlorine activation was significant and large until the beginning of March, peaking mid February. In the middle of March the chlorine activation steeply decreased and ended around 20 March, when the polar vortex broke up. The duration and the magnitude of the chlorine activation in the Arctic winter 1999/2000 were higher than during all previous Arctic winters since the launch of the GOME instrument in April 1995.