Observations of water vapour and temperature from the Earth Observing System Microwave Limb Sounder are used to study dehydration in the 2004 and 2005 polar vortices. Significant differences were found for the Antarctic winters, with the 2005 vortex colder and more extensively dehydrated. For the 2005 winter water vapour reductions were observed from mid-June, coincident with a decrease in temperatures, extending vertically between ~12--21 km. Reductions of up to ~3 ppmv in water vapour were recorded a month later. Permanent dehydration was apparent between ~15--20 km, where 3 months later the temperature recovery was not followed by a recovery in water vapour. The 2004--2005 Arctic winter was unusually cold, but only one single event of depleted water vapour at the end of January was linked to ice formation. For this event, a reduction of up to ~0.5 ppmv was observed over Spitsbergen between ~12--20 km.