An instrument is described with which the relative permittivity and loss of disc samples of dielectric materials may be measured over the frequency range 100–300 Mc/s.The measuring cavity is essentially a short-circuited coaxial transmission line, terminated by the sample capacitance. The line length is adjustable, so that each sample may be measured at a number of different frequencies.All measurements are based upon audio-frequency calibrations of the measuring capacitors, and the design allows these to be used with only small corrections at 300 Mc/s. The corrections are analysed, and a comparison is made of different methods used for the excitation of measuring cavities.The accuracy is estimated to be ±1% in relative permittivity, and ±1½% ±0.00002 in tan δ.