FTIR-spectroscopy has been applied to study model surfaces and processes on cold surfaces which are related to the role of polar stratospheric clouds (PSCs) in O3 depletion in polar regions. Thin solid films of nitric acid hydrates were prepd. by deposition of gas-phase HNO3 and H2O on a cold substrate. Hydrates of HNO3 were the major components of PSCs (type I). In our studies, the mono- and the dihydrate as well as the 2 modifications of nitric acid trihydrate, a- and b-NAT, were obtained and characterized by their FTIR-spectra. The heterogeneous reaction between gas-phase H2O and solid, ionic N2O5 leads to the formation of both the a- and b-modifications of NAT. Gas-phase HBr is hydrolyzed on the surface of a water ice film resulting in the formation of amorphous and cryst. hydrogen bromide monohydrates.