Dielectric properties of saturated, porous geologic materials reflect the large difference in dielectric constant epsilon of typical saturating fluids such as water (epsilon = 78) or oil and gas (epsilon = 1-3). The deconvolution of in-situ dielectric properties of saturated porous materials into the component parts requires a detailed model of the composite material. A model is examined in which the dielectric constant of the composite is equal to the sum of the dielectric constants of the components weighted by the volume fraction occupied by each. That model is compared to measurements at microwave frequencies made on systems consisting of glass beads, quartz, or sand saturated with chlorobenzene, 1, 2-dichloroethane, methanol, or air, and find satisfactory agreement. When water is the saturant an interaction between water and the solid matrix has an important effect on the composite dielectric constant. This interaction is observed to be particularly large for quartz and water and suppresses the composite dielectric constant quite considerably. This interaction is dependent upon the relative surface area per unit volume. An empirical relationship between the surface area and composite dielectric constant is obtained for clean, saturated, unconsolidated reservoirs. The inverse process of determining surface area from in-situ measurements of the composite dielectric constant may be possible for clean reservoirs of known lithology. In sandstones from cores the dielectric constant is also below the volume fraction model and corrections are needed to evaluate water content.--Modified journal abstract.