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Formation-resistivity measurements in boreholes require accurate knowledge about the electrical properties of drilling fluid to exclude any measurement misinterpretations. For many drilling applications, oil-based mud is the preferred drilling fluid because of properties that include excellent shale stability, corrosion inhibition, lubricity, reusability, resistance to contamination, and higher rate of penetration. Although the electrical resistivity of oil-based mud is generally high, it depends on many parameters. We studied electrical resistivity and dielectric permittivity of oil-based muds as a function of frequency and temperature with varying oil and salt content and varying oil/water ratio. Measurements were performed by use of a coaxial test fixture and an impedance analyzer. The accuracy of the measurement system has been determined using standard materials such as air, deionized (DI) water, methanol, nbutanol, and n-octanol. The analysis shows that published models dealing with electrical properties of rocks can be compared well with those of oil-based muds. Resistivity and relative permittivity decrease with increasing temperature and frequency. No clear relationship is revealed between resistivity and water content, but relative dielectric permittivity does depend on water content of oil-based muds. Oil and salt content are of secondary importance. The study shows that the relative dielectric permittivity is related exponentially to the oil/water ratio of oil-based muds, which provides a means of estimating dielectric permittivity of oil-based mud if the oil/water ratio is known.