Electron‐emission density for LaB6cathodes is compared with that for other common cathode types. Equations for describing the poisoning mechanism are presented. Poisoning is related to the gas pressure, cathode temperature, and heat of adsorption. Posioning was measured with a flowing gas system which maintained a constant pressure in the vicinity of the cathode for varying adsorption rates. Poisoning rates were determined by the cathode temperature and gas pressure. An equilibrium poisoned emission level was reached within a few minutes for each cathode temperature and gas pressure. Poisoning gases used were oxygen, carbon dioxide, air, hydrogen, nitrogen, and argon. Oxygen was the most active poisoning gas and argon the least active. No poisoning results until a critical pressure is reached; the emission then decays rapidly with increasing pressure. Resistance to poisoning increases with increasing cathode temperature. At a cathode temperature of 1570°C, the critical poisoning pressure for oxygen is about 5×10−5Torr. At 1400°C, the critical poisoning pressures are as follows: 2×10−6Torr for oxygen, 2×10−5Torr for carbon dioxide, 5×10−5Torr for air, and greater than 10−2Torr for hydrogen, nitrogen, and argon. On a comparative basis, the critical poisoning pressure for LaB6cathodes is several orders of magnitude higher than the poisoning pressure for oxide and impregnated cathodes.