Denitrification is well known to affect the severity of springtime ozone depletion in Polar Regions. In winter 1996/1997 in the Northern Hemisphere and winter 1997 in the Southern Hemisphere, the Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) detected denitrification in both hemispheres. Here the Match technique and a Lagrangian model are used to analyze the nitric acid variation between a pair of measurements belonging to the same air parcel. Eleven cases are studied in Antarctica and seventeen in the Arctic, permitting testing of the laboratory-measured homogeneous freezing rate of liquid ternary aerosol into nitric acid hydrates, thought to be the determining step of the denitrification process. Over the Antarctica, eight cases out of eleven lead to results in agreement with the measurements, taking into account uncertainties of the measurement and possible acceptable bias in the temperature field used for the modeling study. Over the Arctic, more cases remain unexplained even after taking into account the possible scavenging of nitric acid by large nitric acid trihydrate (NAT) particles falling from the layers above. These disagreements are mainly due to relatively high temperatures along the trajectories that do not lead to significant NAT or nitric acid dihydrate (NAD) freezing. Thus it appears that some additional mechanisms are required to explain the denitrification in the Arctic winter. Moreover, the occurrence of denitrification due to the few NAT particles with very large radius (few ¿m), the so-called NAT rocks, over the Antarctic winter in 1997 is suggested.