The structure and photoluminescence of Ca 1؊x La x S (x)3.0-0؍ were investigated. The samples were prepared by sulfurizing the mixture of CaCO 3 and La 2 O 3 in the flux of Na 2 CO 3 -S or K 2 CO 3 -S, and by gas reaction with CS 2 . By using different methods of preparation, defect structure and concentration could be controlled chemically. Ca 1؊x La x S prepared in the Na 2 CO 3 -S flux has a wider solid solution range than that prepared in the K 2 CO 3 -S flux, and such a feature seems to be due to similar ionic size of Na ؉ and Ca 2؉ . As the substitution of La 3؉ increases, the band gap of the host material decreases due to the increase of the lattice parameters, and the photoluminescence spectra of the Ca 1؊x La x S shift to longer wavelengths. Since La 3؉ ion itself is transparent to ultraviolet radiation, vacancies (V Ca 2؉ , V S 2؉) and substituted ions (Na ؉ Ca 2؉ , La 3؉ Ca 2؉) seem to be associated with luminescence centers. The acceptor levels of Na ؉ Ca 2؉ and V Ca 2؉ are estimated to be about 1.1 and 0.5 eV above the valence band, respectively, and the donor levels of La 3؉ Ca 2؉ and V S 2؊ to be about 1.8 eV below the conduction band. The emission bands observed at 500-580 nm suggest the recombination processes of donors with acceptors.