The acid ionization of HBr on ice is important in connection with the rapid springtime ozone depletion in the Arctic tropospheric boundary layer. The interaction of HBr with an ice surface is studied theoretically, via electronic structure calculations on a semiconstrained HBr.(H2O)(13) model cluster at the MP2 level of theory using effective core potentials for all the heavy atoms. The heterolytic dissociation of HBr to form a H3O+.Br- contact ion pair, is not found to occur atop this model ice lattice system, but it is found to be barrierless and energetically spontaneous when assisted by a single extra-lattice water molecule.