A four-electrode setup has been used to measure the complex impedance of partially saturated Berea sandstone and Ottawa sand-bentonite mixtures saturated with n-decane and NaCl brine solutions. The effects of water saturation, clay content, and wettability on both the real and imaginary parts of the rock impedance are investigated at frequencies between 10 Hz and 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. The reactivity index exponent is related to Archie's resistivity index exponent. This relationship can be used to estimate water saturations at frequencies above 0.1 MHz where the resistivity index exponent (used in the Archie equation) is found to be inadequate. Both exponents are shown to increase with wettability and decrease with clay content. The dielectric constant appears to increase linearly with water saturation. The rate of increase of dielectric constant with water saturation is smaller for higher frequencies. The experimentally observed trends are explained qualitatively on the basis of a generalized Maxwell-Wagner theory that accounts for double layer dielectric dispersion.