Desirable properties of potential electron emitter materials include a high melting point, low vapor pressure and compound stability at elevated temperatures, in addition to a favorable work function φ and Richardson number. The rare-earth hexaborides combine many of these properties. We have evaluated and intercompared these properties in an attempt to establish groups of materials desirable for useful thermionic electron emitters. We find that La, Pr, and Nd hexaborides exhibit the most desirable combination of the above properties. Compounds of Pr or NdB6 with 10%–30% additions of LaB6 appear most promising as future emitter materials.