A commercially available blue emitting ZnS:Ag,Cl phosphor was surface-treated with a combination of stirring and ultrasonication in KOH solutions. The stirring and ultrasonication (SUST) enhanced the cathodoluminescence efficiency by 58%. From surface analyses, it was confirmed that OH- ions were physisorbed on the surface of the phosphor, and they formed an additional electrical field on the surface into the E-beam induced field present in cathodoluminescent mode of operation. We observed that the maximum increase of phosphor efficiency at low applied voltages was around 800 V, that the recombination rate of the nonequilibrium charge carriers increased over two times, and that the diffusion-drift length was reduced after SUST. This can be explained by the formation of the increased electrical fields induced by physisorbed OH- ions on the phosphor surface. (C) 2004 The Electrochemical Society.