The role of hydroxide ions in proton mobility in water ice I is studied using both computational methods and isotopic exchange experiments. A strong influence of base adsorbate at the ice nanoparticle surface on proton activity is observed experimentally. Trace doping of the ice surface with base adsorbates stops proton activity, whereas the activity is restarted if larger amounts of adsorbate are present. On the basis of density functional theory calculations for ice slabs, these observations are rationalized by the existence of traps strongly binding hydroxide ions both at the surface and in the ice interior. These strongly binding sites stop proton mobility by immobilization of hydroxide ions; however, in the presence of an abundance of OH-, the deepest traps are saturated and thus the proton activity is restored. A rather broad energy distribution is calculated for the hydroxide located at different lattice sites.