Twoand four-electrode setups have been developed to measure the complex impedance of partially saturated Berea sandstone, tight-gas-sand rocks, and Ottawa sand-bentonite mixtures saturated with n-decane and NaCl brine solutions. The effects of water saturation, clay content, and frequency on both the real and imaginary parts of the rock impedance are investigated between 10 Hz and 10 MHz. A reactivity index exponent (analogous to the resistivity index exponent) is defined and shown to be related to the water saturation by an Archie-type relation. A linear relationship between the resistivity index exponent and the reactivity index exponent has been observed experimentally at frequencies below 0.1 MHz and has been deduced algebraically as well. The reactivity index exponent can be used to estimate water saturations at frequencies above 0.1 MHz where the resistivity index exponent is found to be inadequate. A generalized Maxwell-Wagner model, which accounts for double-layer dielectric dispersion, is used to explain the experimentally observed trends. As the clay fraction is increased by a factor of 2, the dielectric constant of a simulated sand pack is found to increase by approximately the same factor at almost all frequencies from 10 Hz to 10 MHz. This linear dependence between rock dielectric constant and clay percentage has also been observed with the two-electrode data for tight-gas-sand samples of similar porosities and permeabilities.