We investigated the dielectric properties of various organic solvents and binary solvent mixtures at 21.4 degrees C over the frequency range of 200 MHz-13.5 GHz. These solvent mixtures-nitrobenzene -N,N-dimethylformamide, 1-butanol-formamide, nitrobenzene-toluene, ethanol-1-butanol, and nitrobenzene-chlorobenzene-as well as the pure components display a Debye or near Debye dispersion. Their frequency-dependent dielectric properties can be summarized by three parameters in the Debye equation; a static dielectric constant, a high-frequency limiting dielectric constant, and a dielectric relaxation time constant. On the basis of electrostatics, rate theory, and solution thermodynamics, we have developed dielectric "mixing rules" that describe the frequency-dependent dielectric properties of the solvent mixtures based on solution composition and the dielectric parameters for the solution components. These "mixing rules" yield good agreement between the predicted and experimental dielectric properties for the binary solvent mixtures over all solution compositions.