Cyclonic eddies in the Antarctic Circumpolar Current produced bybaroclinic instability of the Antarctic Polar Front contain cold cores ofWinter Water centred at a depth of about 250 m in the upperthermocline. Observations of two eddies during the Polarstern EIFEXCruise ANTXXI/3 identified one as a youthful feature with aheterogeneous core and the other as a mature structure with a morehomogeneous core. Observations of changes in temperature in thecore of the latter over 40 days have allowed estimates of vertical andhorizontal diffusion coefficients to be made. These have then beentested for plausibility in model calculations using observed data toestimate the rate of change of the core structure. In particular therelative importance of diapycnic and isopycnic mixing has beeninvestigated by looking at changes in TS relationships. This appearsto show that isopycnic mixing is more likely to lead to the observedthermohaline structures than diapycnic mixing.