The treatment of the surface of an alumina insulator with a Mn/Ti coating significantly increases its voltage‐holdoff capability. Insulators treated with this coating had vacuum‐holdoff voltages about 25% higher than did untreated insulators. During processing (quasimetallizing) the coating penetrates into the alumina, so it is fairly insensitive to damage by abrasion or electrical breakdown. The quasimetallized coatings is also comparable with subsequent metallizing and brazing of the alumina insulator. We conclude that the coating (1) decreases the surface resistivity of the insulator, (2) decreases the insulator’s secondary‐electron‐emission yield, and (3) makes the surface of the insulator dielectrically more uniform.