The impedance of the oxide-coated cathode of disk seal triodes has been measured at frequencies from 730 to 2360 Mc/s and with cathode temperatures from 1250 to 1400° K. The main feature of the method is the use of a photocell for indicating constancy of cathode temperature. Changes of 0.5° K could be measured. The behaviour of the coating is consistent with an equivalent circuit consisting of the grid-cathode capacitance C₁ in series with the coating resistance R.Measurements of the d.c. coating resistance have also been made and the values agree approximately with those obtained at radio frequency. Values of R correspond to specific conductivities of between 0.3 and 0.4 Ω^-1 m^-1 at a cathode temperature of 1250° K, and R is found to decrease with temperature.Finally a method is outlined which utilizes the change of heater resistance with cathode temperature to determine the power dissipated in the coating under oscillatory conditions.