Optical interference techniques were used to measure the real index of refraction of nitric acid/ice films representative of type I and type II polar stratospheric clouds (PSCs). Possible candidates for type I PSCs include amorphous HNO 3 /H 2 O mixtures as well as crystalline nitric acid trihydrate (NAT), dihydrate (NAD), and monohydrate (NAM). Amorphous and crystalline model PSC Films were grown in vacuum by vapor deposition on single-crystal Al 2 O 3 substrates at low temperatures. The real indices of refraction at λ=632 nm were measured for these films using the time-dependent optical interference during film deposition