Single crystal LaB6has become an important cathode material for a variety of electron beam systems. At the typical operating pressures of many such systems interaction of the cathode with the residual gases present can cause radical enhancement of the cathode volatility with concomitant change in the usual conical shape, oxygen‐bearing gases being the most active in promoting such effects. In this paper we discuss a systematic investigation of the variation of volatility and morphology of heated LaB6cathodes with oxygen pressure. The results of this study show that at a low operating temperature of 1600 K the volatility enhancement due toPO2=1×10−6Torr is almost a factor of 100 relative to operation at 1×10−8Torr or less. In contrast, at a cathode operating temperature of 1900 K the enhancement due to the same oxygen pressure is negligible. The volatility enhancement of LaB6forPO2=1×10−7Torr andT=1700 K is only a factor of 2. The volatility enhancement was observed to vary with crystallographic direction and is primarily responsible for the well known formation of facets on an initially conical emitter structure. From the relative rates of growth of the various facets we were able to conclude that crystal face anisotropy of the oxygen enhanced volatility of LaB6increases in the order (110)<(111)<(100). In the case of a truncated conical cathode the results show the expected increase in rate of change to a pointed pyramid final shape as the cone angle is decreased from 90° to 70°.