an x-band microwave bridge-interferometer is constructed for measuring the permittivity of solids. using simple theory, the system is first tested with several materials of known permittivity (e.g. ertalon, plexiglas and teflon). it is found that the results agreed well with published data for lossy materials only. loss factors of very low-loss materials are irregular rather than linear with the sample lengths as is expected. this is due to the fact that the usual simple theory neglects internal multiple reflections which are predominant inside low-loss materials. the problem does not exist in lossy materials because the transmitted signal is sufficiently dissipated through one traverse of the sample. theoretical correction for low-loss materials is made but is found too tedious to be used. on the other hand, multiple-reflection effects could be eliminated if the samples are cut exactly to multiples of half-wavelengths. samples of arbitrary lengths have to be used first to obtain a preliminary measurement of the real part of the permittivity which is related to the wavelength of the sample by a simple equation. fine adjustment of the klystron frequency is also made to ensure minimum reflections by the samples. the precision method is then employed to study the dielectric properties of pure polystyrene and polystyrene doped with diluted amounts (0.1~2.0%) of polar molecules (viz. 8-quinolinol) at various temperatures (20°-80°c). it is anticipated that the new data obtained here will provide useful information for the study of dielectric relaxation mechanisms in these substances.