A new luminescence band has recently been observed at low temperatures in crystals of CdS and CdSe after the treatment of a crystal surface by various electrolytes. Here we present a theory for this new band and report on its further experimental investigation. Comparison of the theoretical results with experimental data confirms that the new luminescence band can be attributed to radiative recombination of electrons and holes localized in the wells of the potential relief caused by fluctuations of the charge concentration on the crystal surface. This comparison yields a method to quantitatively characterize the density of charged impurities on the surface.