The synthesis of highly efficient, green-emitting SrS:Mn phosphors is described in detail. Cathodoluminescent efficiencies of 110% relative to the commercial TV phosphor, ZnS:Cu, Au, Al, were routinely achieved. Phosphors with the highest luminous efficiencies were obtained with a manganese concentration of 0.15 mole percent which resulted in a saturated emission with a chromaticity of x = 0.325, y = 0.650. The ZnS:Cu, Au, Al standard had x = 0.305, y = 0.615.