The severe Arctic ozone reduction in the winter 2004/2005 is analyzed using ACE-FTS observations and four different analysis techniques: correlations between ozone and long-lived tracers (adjusted to account for mixing), an artificial tracer correlation method, a profile-descent technique, and the empirical relationship between ozone loss and potential PSC volume. The average maximum ozone loss was about 2.1 ppmv at 475 K--500 K (~18 km--20 km). Over 60% of the ozone between 425 K--475 K (~16 km--18 km) was destroyed. The average total column ozone loss was 119 DU, ~20--30 DU larger than the largest previously observed Arctic ozone loss in the winter 1999/2000.