In order to clarify the poisoning mechanism of the LaB6cathode, the temperature dependence of oxidation on a single‐crystal surface was examined by Auger electron spectroscopy. The oxidation gives the chemical effect on the La and B Auger peaks. Evidence from the Auger spectra indicates that the oxygen is more strongly bound to La ions than B ions on the surface. A correlation between the oxidation and the poisoning characteristics was established, and possible reasons for the correlation are discussed.