The dielectric response of geologic materials is related to the amount of water-wetted solid surface present. This dependence raises the possibility that dielectric measurements may be used to detect changes in the state of the solid-fluid interface such as those caused by the sorption of a chemical species. In this study, complex impedance data are collected for systems of clean and oil-bearing sand and kaolinite vacuum-saturated with a 0.001 M NaCl brine in the frequency range 100 kHz to 10 MHz. The effective dielectric constant &kgr; and electrical conductivity &sgr; of the samples are calculated from these measurements assuming quasi-static conditions. We define the components in our system as brine, air, and a wetted solid phase. In this way, the contribution of the solid-fluid interface to the bulk dielectric response is included in the last component. We use an inclusion-based effective medium theory to extract the dielectric response of the wetted solid from the experimental data. The presence of sorbed oil is found to have little effect on the dielectric response of the wetted sand matrix, presumably because of the very low surface area (0.2 m2/g) of the solid-fluid interface. In contrast, the kaolinite (5--12 m2/g) data indicate a decrease of the order of 50% in the dielectric constant of the wetted matrix phase as the amount of water-wetted surface in the sample decreases. The results from this experimental study show that the presence of sorbed oil leads to a detectable change in the dielectric properties of high surface area geologic materials. ¿ 2001 American Geophysical Union