Fourier transform infrared spectroscopy was used to examine the competitive growth of films representative of polar stratospheric clouds. These experiments show that either crystalline nitric acid trihydrate (&bgr;-NAT) or amorphous films with H2O:HNO3 ratios close to 3:1 formed at temperatures 3--7 K warmer than the ice frost point under stratospheric pressure conditions. In addition, with higher HNO3 pressures we observed nitric acid dihydrate (NAD) formation at temperatures warmer than ice formation. However, our experiments also show that NAD surfaces converted to &bgr;-NAT upon exposure to stratospheric water pressures. Finally, we determined that the net uptake coefficient for HNO3 on &bgr;-NAT is close to unity, whereas the net uptake coefficient for H2O is much less. ¿ American Geophysical Union 1992