The ionization of nitric acid on the surface of crystalline ice was examined from 130 to 150 K using FTIR transmission spectroscopy. A spectral feature of the hydronium ion, H 3 O + , was monitored as a function of time. The results are best understood when they are separated into (a) low and (b) high nitric acid exposure, depending upon the amount of nitric acid adsorbed on the ice surface. (a) For low nitric acid exposure ((∼2-20) × 10 15 molecules/cm 2 ), the absorbance of H 3 O + can be fit to a single exponential (i.e., first-order expression). The resulting rate constant, k ) (4.9 ( 0.7) × 10 -3 s -1 , is attributed to the dissolution of the ions in the ice surface layer (i.e., a reactive layer on the ice surface). The thickness of this ice surface layer is estimated to be 10 nm. (b) For high nitric acid exposure ((∼20-200) × 10 15 molecules/cm 2 ), the absorbance of H 3 O + can be fit to a double exponential expression that is composed of the first-order rate constant above, along with another first-order rate constant, k′ ) (5.2 ( 0.7) × 10 -4 s -1 . This rate constant is attributed to the dissolution of ions in an acid-rich ice surface layer. Both rate constants are independent of temperature, indicating a small activation energy (E a ) 0 ( 2 kcal/mol).