Microstructure, acoustic Doppler current profiler, and conductivity-temperature-depth (CTD) profiles were taken in a mesoscale eddy in the Antarctic Polar Front Zone at about 2°15′E, 49°15′S during the R/V Polarstern cruise ANT XXI/3 within the scope of the European Iron Fertilization Experiment in January–March 2004. The mixed layer depth (MLD), calculated from the composite of CTD- and microstructure sonde (MSS)-derived data, was 97.6 ± 20.6 m. No significant correlation between the wind work (E ₁₀) and the MLD (r = 0.02 to 0.22) was found. However, the analysis revealed a negative correlation between the surface buoyancy flux (B) and the MLD 1/2 d later. Two approaches were used to estimate the actively mixing layer depth (AMLD). First, the actively mixing layer was determined subjectively by analyzing the MSS-derived density, energy dissipation, and Thorpe scale profiles, and second, the mixed layer model embedded in a general circulation model was used. The overall mean of the determined depths of the actively mixing layer (AMLD_MSS = 66.4 ± 28.8 m) agreed with the model-predicted boundary layer depths (BLD) (BLD_KPP = 69.1 ± 29.5 m), but the individual values sometimes were differing considerably. We deduced estimates of the vertical diffusivity (K _z ) from the MSS-derived energy dissipation rates and Thorpe scales. Both methods showed that K _z decreased with depth from order of magnitude 10⁻¹ m² s⁻¹ in the actively mixed layer to order of 10⁻⁴ m² s⁻¹ in the pycnocline.