Pyrite occurs in many petroleum reservoirs and significantly affects density and electric well logs even in low concentrations. Ultrahigh-frequency dielectric measurements also are significantly affected by pyrite. Dielectric properties of core samples containing approximately 50 percent pyrite by volume were measured from 800 to 1 200 MHz (10 6 Hz). These measurements demonstrate the high, complex dielectric permittivity of pyrite and provide data for calculation of pyrite permittivity at ultrahigh frequencies.The measured complex-permittivity of water-saturated pyrite nodules is approximately (40-j70)epsilon 0 at 1100 MHz and 25 degrees C. The measured permittivity of pyrite nodules depends upon the concentration of pyrite and only slightly on saturating-water salinity.Two geometric mixing models, the complex refractive-index method (CRIM) and a modified Hanai-Bruggeman-Sen (HBS) model, are used to analyze the data. CRIM analysis of water-saturated and oil-saturated core data gives inconsistent results because the porosity is poorly connected. HBS, with a variable dielectric exponent, provides more consistent results. epsilon * pyrite = (80-j200)epsilon 0 is the complex permittivity of pyrite at 1100 MHz and 25 degrees C determined by applying HBS to laboratory measurements. CRIM-determined pyrite permittivity is about 50 percent greater in both real and imaginary parts.This high epsilon * pyrite has important implications for high-frequency dielectric well-logging tools. Signal amplitudes are highly attenuated in formations with large concentrations of pyrite. Where pyrite nodules occur, wave scattering may cause inconsistent data indicative of formation heterogeneity. Low concentration, disseminated pyrite increases the rock "matrix" permittivity. Mineralogical variations, particularly in trace quantities of heavy minerals, are a likely explanation for significant variations in measured matrix permittivities for sandstones. Comparison of pyrite permittivity with available values for the permittivity of rutile, another heavy mineral found in sedimentary rocks, suggests employing pyrite and rutile in artificial sandpack experiments. The real parts of pyrite and rutile permittivities are nearly equal, but the imaginary part of the permittivity of rutile is lower than pyrite by about 10 5 . Experiments with both minerals could test mixing models and the effects of distribution in mixtures of conductive minerals.