It is shown that a large amount of flicker noise is generated in tubes that have developed a high‐resistance interface layer between the cathode‐nickel and the oxide coating. The equivalent noise resistanceRnof this effect can be represented by the equationRn=KIp2Ri2/f&agr;, whereKis a constant,Ipthe plate current,Rithe interface resistance, andfthe frequency whereas &agr; varies between 1.1 (for lowRi) and 1.5 (for highRi). All evidence suggests that the noise is generated in the interface layer; this was subsequently proved by a direct experiment. The current dependence suggests that the noise is caused by spontaneous fluctuations in the resistanceRi. Experiments up to 1.5 mc indicate a change in the exponent &agr; from 1.1 to 1.6 at high frequencies, indicating that the interface impedance cannot be represented by a fixed resistanceRiand a fixed capacitanceCiin parallel.