Using 7 years of ozone, humidity and temperature radiosoundings performed every two-weeks at Reunion Island (21 ¿S, 55 ¿E), a systematic diminution of ozone mixing ratio at level of tropical cirrus clouds is evidenced. The average depletion of ozone concentration, as compared to adjacent levels, is above 25%. This effect is further documented using concomitant Lidar and radiosounding data. Ice cloud particles evidenced from polarization and backscattering Lidar data are observed at the same level than the ozone depletion in a detailed case study. The ozone concentration is 30% weaker at this level than outside the cirrus cloud. Backtrajectory analyses further indicate that the advection of O3-poor layer injected from lower levels is not likely. ¿ 2000 American Geophysical Union