Crystallographic and optical studies were undertaken on powder samples of LiYO2doped with Eu3+and Tb3+. At room temperature, the compounds are monoclinic. A slight rise of the temperature or small quantities of a smaller cation (Tb4+) 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 Y2O2S:Eu3+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. The7Fmultiplet and the lowest5Denergy levels of the Eu3+(4f6) and the Tb3+(4f8) configurations are determined. Experimentally fitted crystal field parameters are compared to predicted parameters.