We have investigated temporary and irreversible denitrification in the polar regions. We find that the formation of type I polar stratospheric clouds (PSCs) in both hemispheres is best described by a supercooled ternary solution model. Considering the uncertainties in the analysis, our results are also consistent with a nitric acid dihydrate composition for type I PSCs, but not a nitric acid trihydrate composition. In the Northern Hemisphere, we observe that an upper limit of 17% of HNO3 is irreversibly lost each time the temperature of an air mass drops below ~190 K. It seems likely that in order to remove a significant fraction of the HNO3 from an air mass in the Northern Hemisphere, one must expose it to low temperatures multiple times. ¿ 1999 American Geophysical Union