A four day high temporal resolution MU radar RASS data set was obtained under light wind conditions in August 1995, enabling retrieval of virtual temperatures up to 20 km. This provided a unique opportunity to study the vertical eddy diffusivity K across the tropopause using RASS Brunt-Vdisdld frequency N and radar spectral width. Co-located six hourly radiosondes enabled calculation of K using radiosonde N and radar spectral width. RASS K decreased across the WMO tropopause, from (0.39 1 0.05) m2s-1 in the uppermost kilometre of the troposphere to (0.27 1 0.03) m2s-1 in the lowermost kilometre of the stratosphere. The equivalent radiosonde K were (0.43 1 0.06) m2s-1 and (0.31 1 0.07) m2s-1 respectively. The WMO tropopause seems to provide a better definition than the cold-point tropopause in separating the stratosphere from the troposphere when considering the mixing of minor constituents.