Since 1992, more than 150 radiosoundings have been performed from Reunion Island in the Indian Ocean in Southern tropics. On August 25, 1995, tropospheric ozone amounts of over 200 ppbv through the entire troposphere were encountered. Typical values during that period do not usually reach more than 80 ppbv. In order to explain these disturbances, we performed an analysis using satellite and ECMWF data. Results indicate that several mechanisms are working together, including photochemical effect of biomass burning associated with convection, and stratosphere-troposphere exchange near the subtropical jet stream. Based on results shown in this paper, it is difficult to estimate the relative contribution of biomass burning and stratospheric intrusions. Quantitative estimates should stem from other chemical component measurements and from a more detailed dynamical analysis. ¿ 1999 American Geophysical Union