Denitrification of the stratosphere plays an important role in the annual springtime ozone depletion ("ozone hole") over Antarctica. Field measurements suggest that chemical reactions which are responsible for the removal of NOy reservoir species such as N2O5 from the gas phase take place at the cold surface of polar stratospheric clouds (PSC's) and aerosol particles in the stratosphere.PSC's of type I can form 5-7 K above the frost point of ice in the winterly stratosphere and are believed to be particles consisting of a nitric acid hydrate, probably the nitric acid trihydrate (NAT). PSC's of type II on the other hand consist of water ice particles.In order to study reactions on PSC model surfaces in the laboratory we have investigated the different amorphous and crystalline nitric acid hydrates which can be formed at low temperatures. Thin solid films of these hydrates have been prepared and have been characterized by recording their RAIR (Reflection Absorption InfraRed) spectra. It was found that all the different hydrates are metastable concerning irreversible, thermically inducable transformation to beta-NAT, which is one of the two possible modifications NAT can form.We have then studied the reaction of N2O5 with water ice, a PSC type II model surface at low temperatures. Above 150 K reaction takes place which rapidly produces HNO3, which is incorporated into the ice solely as alpha- or beta-NAT, depending on temperature. In the polar stratosphere PSC type II surfaces (water ice) might be converted to PSC type I (NAT) surfaces by this process.