Enhanced BrO and ClO in the boundary layer associated with ozone destruction have been observed over salt lakes, as well as in the polar boundary layer. Volcanic plumes are a major natural source of atmospheric trace gases, influencing the tropospheric and stratospheric trace gas budgets. Though a variety of volcanic gases have been investigated and BrO was found, there is still little information on other halogen oxides (e.g. ClO) in volcanic plumes and the effects on atmospheric ozone. The current belief that volcanic plumes contain ClO has not been quantified to date. Here we report the successful remote measurement of significant amounts of ClO (as well as BrO and SO2) in a volcanic plume from the Sakurajima volcano in Japan, using ground-based multi-axis differential optical absorption spectroscopy during May 2004. Additionally halogen-catalyzed local surface ozone depletion was observed in the vicinity of the volcano.