The low-temperature chemistry of thin films of water ice and the important stratospheric NOy species dinitrogen pentoxide have been investigated in order to spectroscopically characterize one of the principal heterogeneous reactions that occurs on polar stratospheric cloud particles (PSCs). This contributes to stratospheric dentrification. Using reflection-adsorption infrared spectroscopy, we have observed the formation of both covalent and ionic forms of solid N2O5 on both ice and the clean substrate. Thermal evolution experiments suggest that the solid covalent phase, which can only be formed at the lowest temperatures (T<100K), is metastable with respect to the ionic phase. However, we observe no reaction between the solid ionic form and the pure ice film at T<170K. Reaction does occur above 140K between the ice film and gas-phase covalent N2O5 to form a surface layer of hydroxonium ions, solvated nitrate ions, and molecular nitric acid. Annealing to 160K introduces more water into the surface layer by diffusion and produces a further reaction of the excess NO2+ and results in the formation of more molecular nitric acid. Some of the water reacts with molecular nitric acid to form an amorphous hydrate (H2O)(n).H3O+NO3-. From this, we conclude that only N2O5 in the gas-phase covalent form reacts readily with ice and consequently that the reaction in the stratosphere is likely to involve the interaction between the gas phase and the ice surface and not a solid-solid interfacial one. The reaction of gas-phase N2O5 with a nitrate-containing ice surface and the competition between surface species for excess water has important implications for different N2O5 chemistry on type I (NAT) and type II (ice) PSC particles.