It has been widely assumed, without adequate experimental verification, that barium‐strontium oxide, as used in the oxide cathode, is an excess electronic semi‐conductor. Accordingly, the electrical conductivity of (Ba, Sr)O has been studied as a function of temperature before and after activation with methane, extensive precautions being taken to exclude spurious effects. The increase in conductivity obtained characterizes (Ba, Sr)O as a ``reduction'' semi‐conductor, and hence very probably as an electronic semi‐conductor whose conduction electrons arise from a stoichiometric excess of (Ba, Sr) atoms in solid solution.A basic prediction of the semi‐conductor theory has been tested quantitatively with the finding that the electrical conductivity and the thermionic emission of a (Ba, Sr)O cathode are directly proportional through three orders of magnitude of activation; well‐defined chemical and electrical activation and deactivation procedures were used in obtaining this result. It may be concluded that activation represents an increase in the chemical potential of the electrons in the oxide, little or no change in the state of the surface occurring. It has also been found that deviations from the proportionality of conductivity and emission may be expected under conditions leading to inhomogeneity in the oxide, in agreement with the semi‐conductor theory also.