A laboratory apparatus is designed to measure the complex permittivity of both liquid and solid samples in the frequency range from 2 to 13 MHz. This device uses a guarded-electrode sample holder to significantly reduce fringe effects. A synthetic focusing method is utilized to ensure that the potentials at both the main and the guard electrodes are equal. Extensive measurements on NaCl solutions of different salinities are made to study the accuracy of the guarded-electrode method. The measured complex permittivities are in good agreement with those reported in the literature. The deviation is less than 3% for the conductivity measurement and depends on the salinity of the solution for the dielectric constant measurement. Various measurements conducted on Berea sandstones and glass beads saturated with saline water showed the accuracy and feasibility of the experimental set up. This measurement method might be used to obtain accurate values for the conversion of measured data from the measurement while drilling (MWD) resistivity tool.