We present measurements of stratospheric water vapor taken at high northern and southern latitudes by the Polar Ozone and Aerosol Measurement (POAM) III instrument. The measurements show the seasonal variations in both hemispheres, including wintertime descent in the polar vortices and dehydration in the Antarctic. While POAM is unable to measure in regions with high aerosol extinction and can therefore not measure the temporary sequestration of water in ice clouds, POAM can measure the effects of such dehydration events if they result in the permanent removal of water vapor from an air parcel. A study of POAM matches, where the same parcel is observed more than once by the POAM instrument, shows significant permanent dehydration in the Antarctic for nearly all cases in which the parcel is either exposed to conditions where the mixing ratio either exceeds the saturation mixing ratio by >2 ppmv or experiences temperatures that remain below the saturation temperature for >2 days. These POAM matches suggest that parcels undergo several episodes of dehydration before their water vapor mixing ratio is reduced to the 1--2 ppmv level typically observed within the vortex in the Antarctic spring. Interannual differences in Antarctic vortex temperatures are shown to be qualitatively consistent with interannual differences in dehydration observed by POAM. Although analyses of parcels observed by POAM in the Arctic indicate that a few of these parcels experience conditions which generally cause permanent dehydration in the Antarctic, no significant permanent dehydration is observed in the POAM Arctic measurements.