Vertical profiles of N2O5, HO2NO2, and NO2 inside the arctic vortex were retrieved from nighttime infrared limb emission spectra measured by the Michelson Interferometer for Passive Atmospheric Sounding, Balloon-borne version 2 (MIPAS-B2) instrument from Kiruna (Sweden, 68 ¿N) on February 11, 1995, as part of the Second European Stratospheric Arctic and Midlatitude Experiment (SESAME). Spectra were analyzed by a multiparameter nonlinear least squares fitting procedure in combination with an onion-peeling retrieval algorithm. The N2O5, HO2NO2, and NO2 results were derived from spectral features within the bands near 8.0 μm, 12.5 μm, and 6.2 μm, respectively. Peak mixing ratios of 1.14 parts per billion by volume (ppbv) N2O5 and 80 parts per trillion by volume (pptv) HO2NO2 at 17.1 hPa as well as 2.79 ppbv NO2 at 12.0 hPa corresponding to 25.8 km and 28.0 km altitude were inferred from the spectra. NO2 mixing ratios measured by MIPAS fit well to the data observed by concurrent flights. A comparison with calculations performed with a three-dimensional chemistry transport model for the time and location of the measurements shows that the best agreement of measured and calculated profiles is reached between 17 and 28 hPa corresponding to 25.8 and 22.7 km altitude, while below and above this altitude region there are some discrepancies between the modeled and observed data.¿ 1997 American Geophysical Union