Aircraft measurements of ozone, methane, carbon monoxide, relative humidity and equivalent potential temperature were performed during the TROPOZ II campaign. During the descent down to Buenos Aires (34°S, 58°W) at 19:00 UTC on 22 January 1991, a double-peaked layer of elevated ozone was observed. Both a physico-chemical interpretation of each peak and diagnoses based on potential vorticity and ageostrophic circulations indicate the stratospheric origin of the ozone maxima. The relative minimum of ozone and significant increase of relative humidity present in the shallow layer between the peaks, in a region of wind shear, is identified as the signature of real mixing. This intermediate layer shows that ozone originating in the stratosphere was subsequently mixed into the troposphere by folding processes associated with the jet. This result suggests the relative importance of stratosphere-troposphere exchanges to the ozone budget in the Southern Hemisphere, due to a lesser extent of anthropogenic sources in this hemisphere. Copyright (C) 2000 Elsevier Science B.V.Aircraft measurements of ozone, methane, carbon monoxide, relative humidity and equivalent potential temperature were performed during the TROPOZ II campaign. During the descent down to Buenos Aires (34°S, 58°W) at 19:00 UTC on 22 January 1991, a double-peaked layer of elevated ozone was observed. Both a physico-chemical interpretation of each peak and diagnoses based on potential vorticity and ageostrophic circulations indicate the stratospheric origin of the ozone maxima. The relative minimum of ozone and significant increase of relative humidity present in the shallow layer between the peaks, in a region of wind shear, is identified as the signature of real mixing. This intermediate layer shows that ozone originating in the stratosphere was subsequently mixed into the troposphere by folding processes associated with the jet. This result suggests the relative importance of stratosphere-troposphere exchanges to the ozone budget in the Southern Hemisphere, due to a lesser extent of anthropogenic sources in this hemisphere.