A characterization of the NOy partitioning in the warm Arctic winter 1998/1999 is given. Vertical profiles of nocturnal total reactive nitrogen (NOy = NO2 + HNO3 + ClONO2 + 2 N2O5 + HO2NO2) were retrieved from infrared limb emission spectra measured by the Michelson Interferometer for Passive Atmospheric Sounding, Balloon-borne version (MIPAS-B) instrument inside a distortion of the winter Arctic vortex from Kiruna (Sweden, 68¿N) on 27 January 1999. To estimate the dynamic effects, the mixing ratios of the tracers N2O and CH4 were derived to construct a correlation between these two long-lived species and to estimate the effects of mixing on the N2O-NOy relationship. The measured data are compared to calculations performed with the three-dimensional chemistry transport models (CTMs) SLIMCAT and KASIMA. The results show that despite the warm winter 1998/1999 without significant occurrence of polar stratospheric clouds, the agreement between both the measurement and the models and among the models themselves is not satisfactory. It appears that in such a dynamically active winter, mixing processes on different scales which are hard to reproduce with coarsely resolved CTMs should be taken into account to explain the differences. Nevertheless, the results indicate that the chemistry which controls the NOy partitioning is not yet understood well under the studied geophysical situation. Furthermore, chlorine activation appears to be too crudely modeled when winter temperatures are marginal for polar stratospheric cloud formation.