Arctic denitrification in the winter 2004/2005 is analyzed using measurements from the satellite instrument ACE-FTS. Two methods based on changes in the correlations of odd nitrogen (NOy) versus methane and versus an artificial long-lived tracer are employed. The observations show that significant denitrification occurred in the region 15 km--20 km and that it was spatially inhomogeneous within the polar vortex. Before the vortex breakup in mid-March 2005 a maximum denitrification of 8 ¿ 0.8 ppbv, accounting for a relative reduction of over 50%, was observed at ~450 K (~17 km) using both methods. When averaged over the polar vortex for the first half of March, the denitrification at this level was about 4.4 ppbv and 5.0 ppbv from the artificial tracer method and the correlation method, respectively.