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A technique for the measurement of the frequency‐dependent complex dielectric constant ϵ=ϵ’-jϵ‘ of liquids applicable to frequencies up to at least 20 GHz is described. The technique utilizes a coaxial probe dipped into the liquid. The reflection coefficient of the probe was measured using a network analyzer between 45 MHz and 20 GHz. A new de‐imbedding procedure for eliminating the connector and coax mismatches is described, during which the impedance Z(ϵ) of the probe end was determined. The dielectric constant ϵ was determined from Z(ϵ) by modeling the coax–liquid interface as a capacitance. The de‐imbedding procedure, which utilizes three calibrations, directly eliminates the (unknown) fringe‐field impedance. Radiation effects were minimized by using narrow (0.047‐in.) semirigid coax. The technique yields accurate results for ϵ’ and ϵ‘ of liquids such as methanol and water over the entire frequency range up to 20 GHz, and can be used to determine the relaxation spectra of liquid and liquidlike samples.