A possible approach to the development of a low work functionthermionic emitter involves the introduction of shallow donorstates into a matrix having a low electron affinity. It is the intentof this paper to explore the concept of a shallow donor emitterfrom both a general point of view and with specific application tobarium oxide as the host lattice. The problem is discussed in termsof the single-donor model with consideration given the variouscharacteristics which would be required for a practical matrix-additivesystem. Although the electron affinity of BaO has beenestimated to be as low as 0.6 eV, the ordinary oxygen vacancydonor present is a deep level with an ionization energy of about1.4 eV; it yields a work function of 1.4 to 1.5 eV at 1000°K. Bycomparison, a work function of 0.83 eV would be expected forshallow donor BaO at this temperature. The substitution of atripositive rare-earth ion for aBa2+ion in the lattice is presentedas a possible mechanism for the incorporation of an impurity donorcenter. Some speculation is offered concerning the ionizationenergy of this type of donor as well as the associated activationprocess. An attempt was made to observe the donor behavior ofseveral rare-earth ion additives in BaO and also SrO by studyingthe temperature dependence of the effective work function. Datawere secured for La, Gd, Nd, Er, and Eu in BaO, and Eu and Smin SrO at analytical concentrations of from 0.01 to 0.05 mole %While no lowering of the work function is reported, it is clear thatno definitive interpretation of a negative result can be made untilcertain other experimental information becomes available, particularlythe solubility and oxidation state of the additive ion in thematrix crystal.