Data from a series of ozone soundings taken at Uccle during a pronounced polar front passage have been used to study the vertical distribution of ozone in relation to the synoptic situation. The analysis of these data shows a large influx of ozone-rich polar air in the lower stratosphere as the upper-air trough approaches. There is also evidence of an instrsion of stratospheric air into the troposhere in the region of the tropopause break near the polar front jet stream.