A facile oxygen-induced chemisorptive replacement reaction occurs when a Bi(0001)-O overlayer is exposed to hydrogen chloride at 298K. The overlayer, which conforms to the stoichiometry BiOCl2, is stable in contrast to analogous chemistry observed earlier with a Pb(110) surface. Evidence for discrete localized states associated with Bi2+or Bi3+species is observed from shifts in the Bi(4f) binding energies while the charge distribution within the bismuth-chlorine bond is shown to be sensitive to the presence of oxygen adatoms within the oxy-chloride overlayer.