A method is described for measuring the temperature variation of the low frequency fluctuations of emission current (flicker effect) in diodes with oxide‐coated cathodes. In order to vary the cathode temperature and at the same time avoid the effects of space charge and the variation due to changes of the mean current, it is necessary to operate the test diodes under retarding field conditions. The results show that the majority of tubes exhibit minimum fluctuations at a cathode temperature in the region of 1000°K, although both rising and falling temperature characteristics are encountered. It is concluded that the flicker effect is in fact a combination of at least two separate phenomena. Finally, the various theories are examined in the light of the new information, and none is found to be completely satisfactory.