Observations of stratospheric water vapour, made by the Microwave Limb Sounder (MLS) during the 1992 and 1993 Arctic and 1992 Antarctic late winters have now been produced using version 4 of the retrieval software. These improved measurements are analysed as equivalent latitude zonal means. Major interhemispheric differences are revealed in the water vapour content of the vortex in the lower stratosphere. This technique emphasizes mixing ratio gradients at the edges of both polar vortices, and a local maximum at the edge of the Antarctic vortex. There are some small interhemispheric differences in mixing ratios in mid-latitudes, but they are not strongly related to the dehydration of the Antarctic vortex. A mixing ratio gradient across the interior of the Antarctic vortex at 530K indicates it is not isentropically mixed. A strong local maximum in mixing ratio at the center of the Antarctic vortex in the mid-stratosphere indicates it is not well mixed in the mid-stratosphere also. There is little evidence of significant structure inside the Arctic vortex. ¿ 1998 American Geophysical Union