Abstract: The method for measuring the complex dielectric constant of a substance is based on calculations made in the frequency domain. An open ended rectangular waveguide terminates, at one end, in a non-resonant rectangular cavity, which is then filled with a substance of interest. An electromagnetic wave of known frequency is transmitted through the waveguide and the reflection coefficient at the interface between the waveguide and the cavity is measured by a network analyzer or the like. Another measurement is then made with a slight variation in depth of the cavity or a variation in frequency, and from these two measurements of the reflection coefficient, the complex dielectric constant of the substance of interest can be calculated.