A high-luminance and high-color-purity inorganic phosphor Ca 0.5Sr0.5S:Eu has been developed for the red component in ac thin-film electroluminescent devices using electron-beam evaporation. The influence of the Eu concentration on the luminescence and response characteristic of the Ca0.5Sr0.5S:Eu devices was studied. A high Eu concentration causes not only concentration quenching of photoluminescence and electroluminescence (EL) emission, but also increases the light-emitting response time of the Ca0.5Sr0.5S:Eu EL devices upon the application of an electric field. Decay characteristics of the Ca0.5Sr0.5S:Eu thin films, point out the creation of nonradiative decay channels due to the formation the Eu2+ clusters at high Eu concentrations. The slow light-emitting response in Ca 0.5Sr0.5:Eu EL devices was attributed to the presence of traps generated by these Eu2+ clusters. To understand the nature of these traps, the frequency and temperature dependence of the response time was studied.