A method of measurement of the complex permittivity of a semiconductor, in which a sample is introduced into a rectangular waveguide by means of insulated slots in the broad walls, is discussed. The corrections required to relate the behaviour of such a system to that of an idealised structure, which has a convenient theoretical solution, are described, and details are given of a microwave bridge circuit suitable for the measurement of the propagation coefficient of an inhomogeneously filled waveguide section.