Abstract The structure and photoluminescence of Ca 1− x La x S ( x=0–0.3) were investigated. The samples were prepared by sulfurizing the mixture of CaCO 3and La 2O 3in the flux of Na 2CO 3–S or K 2CO 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 2CO 3–S flux has a wider solid solution range than that prepared in the K 2CO 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.