The valence of lanthanum on the surface of carburized La 2O 3 Mo cathode material has been studied using high temperature X ray photoelectron spectroscopy(XPS). The experimental results show that La 2O 3 in the La 2O 3 Mo cathode material can be reduced to metalic lanthanum by molybdenum carbide (Mo 2C). The binding energy of metallic lanthanum is higher than that of La 2O 3, The La3d5/2 binding energy of metallic lanthanum has been proved to be 835 96 eV experimentally. This paper reports for the first time that the experimental La3d3/2 binding energy of metallic lanthanum is 852 23 eV .