Permittivity values of a co-solvent system decrease with increasing temperature and decrease as mole fraction of water decreases. Fit to a cubic polynomial, the coefficients can be used to determine relative permittivity of the co-solvent system knowing temperature and mole fraction. A Brookhaven Instruments BI 870 Dielectric Meter was used. Pure water from 15.0 to 55.0 °C was measured to validate the method. Solutions of acetone and tetrahydrofuran were measured from 15.0 to 55.0 °C. A cubic polynomial fit was used to determine the polynomial coefficients for determining permittivity as a function of temperature and mole fraction. These coefficients may also be used to interpolate the dielectric constant between temperatures. Literature values for acetonitrile, 1,4-dioxane, methanol, ethanol, isopropanol and N,N-dimethylformamide in water were used to determine the polynomial coefficients.