Chemical ozone loss rates inside the Arctic polar vortexweredetermined in early 1998 and early 1999 by using the Match techniquebased on coordinated ozonesonde measurements. These two wintersprovide the only opportunities in recent years to investigatechemical ozone loss in a warm Arctic vortex under thresholdconditions, i.e., where the preconditions for chlorine activation,and hence ozone destruction, only occurred occasionally. In 1998,results were obtained in January and February between 410 and 520 K.The overall ozone loss was observed to be largely insignificant,with the exception of late February, when those air parcels exposedto temperatures below 195 K were affected by chemical ozone loss. In1999, results are confined to the 475 K isentropic level, where nosignificant ozone loss was observed. Average temperatures were some8 -10 K higher than those in 1995, 1996, and 1997,when substantial chemical ozone loss occurred. The results underlinethe strong dependence of the chemical ozone loss on thestratospheric temperatures. This study shows that enhanced chlorinealone does not provide a sufficient condition for ozone loss. Theevolution of stratospheric temperatures over the next decade will bethe determining factor for the amount of wintertime chemical ozoneloss in the Arctic stratosphere.