Using probe diodes, changes in thermionic emission and conductivity of cathodes of calcium oxide on nickel have been followed during activation and poisoning. The effects of material liberated from the anode have been studied. The mean nominal thermionic work function is 1.69 eV for the fully activated cathode (from the slope of a Richardson line), with an estimated temperature coefficient of 1.1 mV/° K.Three regions of conductivity have been found; the activation energies for the two at the lowest temperatures are 1.3 eV and 0.81 eV. The current-voltage characteristics between the cathode base and probe show two types of curvature, one dominant above 625° K and the other below this temperature. A linear relation extending over four orders of magnitude between the logarithms of emission and of the conductivity has been observed during activation. Assuming that semi-conductor theory may be applied the mean value of the surface work function is calculated to be 0.7 eV.