The paper describes a resonance method of measuring permittivity and power factor which is essentially a development of the Hartshorn and Ward apparatus, suitable for use at the higher frequencies. The method is one of capacitance variation, and the disc form of sample, the circular plate electrodes and the cylindrical form of micrometer capacitor are retained but reduced in linear dimensions. The capacitive elements are mounted in a re-entrant cavity, and the micrometer capacitor is so placed that the voltage across it is only a fraction of that across the plate electrodes; this increases the fineness of adjustment. The micrometer capacitor can be calibrated only by using the cavity as a wavemeter; the capacitance settings were therefore expressed in terms of the corresponding resonance frequencies, and the calculations of power factor then became identical in form with those of the frequency-change method, although the measurements were made at a constant frequency. After applying the appropriate corrections the permittivity can be determined to within ± 1% over most of the range. The high Q-factor of the cavity and the precision of adjustment of the micrometer capacitor make the power-factor determinations highly accurate. For values less than about 0.01 the error is estimated as being less than ± 2 × 10⁻⁵ (for measurements on solids). For higher values a superheterodyne receiver incorporating a piston attenuator is used as the detector, and the estimated limits of error for such measurements are ± 5 × 10⁻⁴.