A series of laboratory experiments, including measurements of electrical properties, permeability, and porosity, were performed on saturated sand-clay mixtures. Different mixtures and packing geometries of quartz sand and 0 to 10% Na-montmorillonite clay were investigate using solutions of CaCl2 and deionized water. Two main regions of electrical conduction exist: a region dominated by surface conduction and a region where the ionic strength of the saturating fluid controlled conduction. For low fluid conductivities, the sample geometry was found to greatly affect the magnitude of the surface conductance. The influence of the microstructural properties on the electrical properties was quantified by estimating formation factors, &Lgr; parameters, and surface conductances. The surface conductances estimated using the theory of Johnson et al. [1986> agreed well with measured values. We suggest that high and low bounds on the expected surface and bulk conductances in a natural system can be derived from the measurements on these artificial geometries. ¿ 2000 American Geophysical Union