Abstract: A method is disclosed for producing an aluminum oxide coated manganese activated zinc silicate phosphor which comprises forming a solution of Al.sup.+3 ions in water with the concentration of the aluminum being from about 0.01 to about 0.30 moles/l, adding manganese activated zinc silicate phosphor to the aluminum solution to form a slurry wherein the amount of phosphor is about 0.7 to about 1.0 moles per liter of slurry with agitation for a sufficient time to result in aluminum ions being adsorbed onto the surfaces of the particles of the phosphor, separating the resulting phosphor with adsorbed aluminum ions from the resulting liquor, drying the phosphor with the adsorbed aluminum ions, firing the resulting dried phosphor at a temperature of from about 750.degree. C. to about 850.degree. C.