In recent years, there has been considerable interest in chlorine-containing trace gases in the atmosphere, particularly in Antarctica because of the relationship between chlorofluorocarbons and the antarctic ozone hole. All chlorinecontaining trace gases, whether produced by human activities or by natural processes, have a potential for destroying ozone in the stratosphere. This is a complex environmental problem, but it is clear that manmade chlorine-containing gases are the driving force behind the antarctic ozone hole and, by extension, reductions of stratospheric ozone over other parts of the world [World Meteorological Organization (WMO) 1989, 1991, 1995]. We have taken measurements of the major chlorine-containing gases in Antarctica for more than 20 years, first at Amundsen–Scott South Pole Station and more recently at Palmer Station (64.46°S 64.04°W). Here we report the results of this work. During recent years, major changes in the concentration of ozone-depleting compounds have taken place in Antarctica because of the Montreal Protocol, which is designed to phase out the production of chlorofluorocarbons and related compounds, to prevent the destruction of the ozone layer (WMO 1995).