A blocking ridge over the Atlantic and N.W. Europe was the most persistent feature of the upper troposphere during the winter of 1991/1992. With brief intermissions, blocking lasted for almost three months. It produced large anomalies in the mean ozone fields for December 1991 and for January 1992; record low amounts of total ozone were reported over Europe. The influence of this blocking system on the structure of the polar vortex is discussed. In the lower stratosphere there was a persistent collocation of the core of the jet stream with temperatures low enough to promote heterogeneous processes, subjecting a large amount of air to chemical processing. However the minor warmings of mid-January and February make it difficult to identify chemical ozone loss against the background of the large anomalies present in 1991/1992. ¿ American Geophysical Union 1994