Capacitance and conductance dispersions and a dissipation factor maximum were observed in single-crystal NaCl specimens at temperatures between 300° and 400°C. Probe measurements of the potential profile across these specimens in a dc field revealed the presence of a large potential drop in a very thin region adjacent to each electrode comparable in magnitude to the linear potential drop throughout the entire bulk of the crystal. The observed effects are shown to be consistent with an interpretation based on interfacial polarization arising from the presence of a surface layer having a higher resistivity than the bulk of the crystal. Interpretations based on dipole rotation or charge-carrier blocking at the electrodes are shown to be in disagreement with the observed polarization data.