Molecular dynamics simulations were used to study the acid ionization of hydrochloric acid (HCl) at the basal plane surface of ice at 190 kelvin, as a model for the acid ionization process in Antarctic polar stratospheric clouds (PSCs). Initial conditions for HCl placement within the top bilayer of the ice lattice were selected on the basis of relevant dynamic equilibrium adsorption-desorption conditions. Free energy changes calculated for the first step in the stepwise acid ionization mechanism ranged from -5.8 to -6.7 kilocalories per mole for various likely initial conditions. These results indicate that acid ionization is thermodynamically favorable and that this process has important implications for ozone depletion mechanisms involving PSCs.