We describe, for the first time, measurements of stratospheric nitrogen dioxide (NO2) by the Polar Ozone and Aerosol Measurement (POAM II) instrument. Measurements span October 1993 through mid-November 1996 and cover latitude ranges from 55¿ to 72 ¿N and from 63¿ to 88 ¿S. Comparisons with coincident satellite and space shuttle observations show good agreement and confirm the validity of POAM II measurements for scientific investigations. Overall seasonal variations in both hemispheres are qualitatively consistent with standard photochemistry. In the austral late winter/early spring of 1994, however, anomalously high NO2 mixing ratios were observed above 22 km. We conclude that these high NO2 levels resulted from downward transport of NOx-enhanced air from the mesosphere or thermosphere inside the polar vortex. Enhanced NO2 mixing ratios in 1994 were factors of ~1.3 and 2.5 larger than at the corresponding times in 1995 and 1996. We conclude that POAM II observations of coincident, localized reductions in ozone of up to 40% were caused by the increased stratospheric NOx via the standard catalytic NOx cycle. ¿ 1998 American Geophysical Union