Polar Stratospheric Clouds (PSC's) play an important role in spring-time stratospheric ozone depletion in particular in Antarctica. PSC's are formed at sufficiently low temperatures (<-80°C) in the winterly stratosphere of the polar regions. On the surfaces of the PSC particles heterogeneous chemical reactions can take place which are significant for stratospheric ozone chemistry.Type II PSC's are composed of water ice, while the most frequently observed form of PSC's (type I) consists of nitric acid and water, probably in the form of crystalline nitric acid trihydrate (NAT). However, HNO3 and H2O can generate various forms of nitric acid hydrates.By means of the matrix isolation technique we have prepared solid films consisting of water and nitric acid. We have been able to identify nitric acid monohydrate (NAM), dihydrate (NAD) and two forms of the trihydrate (alpha- and beta-NAT). The solid surface films -representing PSC surfaces - have been characterized by means of RAIR (Reflection-Absorption-InfraRed)spectroscopy.We have also studied heterogeneous reactions on such surfaces, in particular with HBr and N2O5. For example, the reaction of N2O5 with ice surfaces gnerates HNO3 which remains in the solid phase. Depending on temperature HNO3 is transformed into NAT either in the alpha- or beta-form. The heterogeneous reaction of HBr with solid nitric acid yielded BrNO as one of the new reaction products.