For over twenty years, in the course of our work on the oxide-coated cathodes, we have frequently come across many phenomena which can not be explained by the semi-conductor theory. This has led us to re-examine carefully most of the parameters, such as work functions, electrical conductivities, spectral absorption, concentration of excess barium, lattice structure,surface energy states, distribution of potentials in the coating, comparsion of thin film emission to coating emission, non-uniformity of emission, flicker noise, instability of impulse current, sparking etc. All these show that the oxide-coated cathode is not an n-type semi-conductor with excess barium as donor. A preliminary theory of surface emission centers which can explain many phenomena in the oxidecoated cathode is proposed.