Surface studies of LaB6 cathode failure has been carried out by using scanning Auger mi-croprobe model PHI-590. Results show that the failure mechanism is related to the severe containmina-tion of carbon and oxygen penetrating deep into the bulk and the relative inadequacy of Lanthanum atoms to Boron atoms. Two possible containminatlon sources are gas phase carbides in the vacuum chamber and surface diffusion of graphite which is the constituent of LaB6 cathode module.