For understanding solvation by ionic liquids, it is mandatory to characterize their static relative dielectric permittivities epsilon ("static dielectric constants"). Exploiting the definition of E in terms of the zero-frequency limit of the frequency-dependent dielectric dispersion curve, the static dielectric constant of an electrically conducting liquid can be extrapolated from dielectric relaxation spectra in the microwave regime. On the basis of this method, we report dielectric constants of 42 ionic liquids at 25 degrees C.