Although the difficulties involved in the measurement of dielectric properties increase with frequency, a conventional cavity method with minor modifications has proved satisfactory at millimetre wavelengths.A cavity, ½in in diameter, was excited in an H01mode by means of coupling holes. It was found to have sufficiently low wall loss to enable the loss tangents of disc specimens of magnesium-titanate and titanium-dioxide ceramics to be measured. Both loss tangents were of the order of 0.0003, and the relative permittivities were 14 and 80, respectively.Although a high degree of accuracy is not claimed, useful results can be obtained with relatively simple equipment. A convenient method of marking small frequency intervals is described.