A method is given by which resonance-absorption isolators may be designed to handle any required mean power level and load mismatch without deterioration in performance. The method is derived from a study of the properties of small discs of ferrite in X-band waveguide at high mean powers and of the thermal conditions in the ferrite. It requires the proper choice of the shape demagnetizing factors in the isolator, together with a knowledge of the maximum temperature at which the ferrite may usefully be operated at gyromagnetic resonance.Practical limits to power handling by this method are discussed, and the relative degrees of usefulness of the various materials measured for this application are presented in terms of their maximum operating temperatures.Typical performance data are given of resonance isolators in the X-band, which will handle mean forward powers of over 2 kW with a 25% load reflection, whilst maintaining their low-power characteristics.