​Changes in the characteristics of the pore fluid in soils, such as concentration, valence, and permittivity, affect the electrical properties of the bulk fluid and the formation of double layers, causes volumetric changes and alters the fabric of the soil. The complex permittivity spectrum of a clay-fluid system reflects the polarizability of the phases and their interaction. This paper presents a review of polarization mechanisms, followed by high-frequency (MHz-GHz) experimental results involving two clays of very different specific surface, aqueous electrolytes with varied concentration and valence, and non-aqueous phase liquids NAPLs. Complex permittivity measurements reveal the effect of mixing order, both in the case of electrolytes and when the clay-fluid mixture includes organic liquids. The development of double layers is hindered when NAPLs reach clay surfaces before water. Various processes that influence the complex permittivity of geomaterials are identified. Results are relevant to the interpretation of field measurements with ground penetrating radar and time domain reflectometry.