The rare‐earth oxysulfide La10S14O, doped with cerium, emits a bright luminescence band peaking around 6350 Å at room temperature. This red emission is quenched by copper impurities inadvertently present in insufficiently purified La10S14O. The Cu impurity in turn brings forth an emission band peaking around 7200 Å. The red 6350‐Å luminescence has technological importance for color TV screen and luminescent lamp application. We report on the photoluminescence properties of doped La10S14O and on refinements of its structure determination.