The donor behavior of various additives in barium oxide can be conveniently studied by observing the variation of the effective thermionic work function with temperature. The erivation of both the onizatio energy and density of the donors is treate in erms of a static singledonor mod l the acc ptor concentration is assumed to be reduced to zero by activators in the base nickel. A demountable est diode structure was assembled which accommo tes interc angeable samples prepared by electrodeposition. Experimental results were obtained for pure barium oxide, b rium oxide con aining 0 025 mole percent lanthanum oxide, and barium oxide containing 0.035 mole percent gadolinium oxide. No shallow level formation was observed for these additives the empirical ionizatio energies and densities are discussed.