Abstract Crystallographic and optical studies were undertaken on powder samples of LiYO 2 doped with Eu 3+ and Tb 3+ . At room temperature, the compounds are monoclinic. A slight rise of the temperature or small quantities of a smaller cation (Tb 4+ ) induces a phase transition to a tetragonal form. The radiant efficiencies are 0.33 and 0.47 those of the commercial red and green TV standards Y 2 O 2 S:Eu 3+ and ZnS:Cu,Al, respectively. While stable at room temperature, the compounds do decompose into the oxides as the temperature is raised above 250°C. The 7 F multiplet and the lowest 5 D energy levels of the Eu 3+ (4 f 6 ) and the Tb 3+ (4 f 8 ) configurations are determined. Experimentally fitted crystal field parameters are compared to predicted parameters.