We have measured the dielectric constant and electrical resistivity of Topopah Spring tuff as a function of saturation at 23¿ and 40 ¿C. Dry and saturated resistivities can differ by more than 4 orders of magnitude. Frequency-dependent impedance measurements indicate that the overall electrical response of the rock arises from at least three conduction mechanisms, which we tentatively identify as conduction through adsorbed water on solid surfaces, conduction through isolated pockets of water, and conduction through continuous (bulk) water. The relative contribution of each mechanism varies with saturation. A continuous (bulk) water film is indicated at saturations as low as 35%. Reasonable agreement was found between predicted permeability based on electrical measurements and microstructural parameters using the Kozeny-Carmen relation and experimentally determined permeabilities. Similarly, good agreement was found between estimates of cation exchange capacity (CEC) based on electrical properties through the Waxman and Smits <1968> equation and published CEC values for tuff.¿ 1997 American Geophysical Union