Two years of ozone measurements (1996, 1997), sorted according to the analyzed potential vorticity over Dumont d'Urville (66 ¿S), are presented. Subsidence in the vortex was observed by tracing the descent of the ozone maximum. Chemical depletion was also observed. In spring-time, the ozone mixing ratio was reduced by a factor 10 on isentropes between about 400 K and 475 K. Using the observations of descent and depletion, it is shown that, at the latitude of Dumont d'Urville, chemical depletion starts close to midwinter. Depletion starts earlier at higher altitudes (above 450 K) than at lower altitudes (below 450 K). The earlier presence of Polar Stratospheric Clouds (PSCs) at higher altitudes than at lower altitudes is a likely reason. The mid-winter maximum in Antarctic total ozone columns [Roscoe et al., 1997> is then due to the integrated effect of these phase-shifted maxima on the various isentropic surfaces. ¿ 1999 American Geophysical Union