We examined the interaction of HNO3 with water ice for partial pressures 2 × 10(-8) Torr to 1 × 10(-5) Torr and at temperatures from 216 to 256 K using (i) the surface-specific technique ellipsometry and (ii) a coated wall flow tube reactor, both coupled with chemical ionization mass spectrometry detection of HNO3 in the gas phase. Our ellipsometry results show that exposure to HNO3 induces surface disordering on ice at a range of environmentally relevant temperatures and HNO3 partial pressures, particularly in the vicinity of the boundary between the ice and the HNO3·3H2O phases. The coated wall flow tube studies indicate that the nature of HNO3 uptake changes from reversible adsorption to a continuous flux of HNO3 into the bulk in the presence of a disordered interfacial layer. These results have implications for atmospheric chemistry in the upper troposphere and in polar regions.