Abstract: A method and apparatus for microencapsulating an electroluminescent phosphor with an extremely thin coating. The process involves vaporizing a poly-para-xylylene, pyrolyzing the resulting vapors to form reactive monomeric radicals, and then passing such vaporous radicals to a vacuum deposition zone where they contact tumbling particles of an electroluminescent phosphor under vacuum conditions. As the particles are being coated, they are sifted by means of a screen. The reactive monomeric radicals deposit on the tumbling particles, are adsorbed by the particles, and polymerized into a very thin coating on the surfaces of the particles.