Precision measurements of the relative permittivity of n heptane at pressures up to 3334 bar and over a temperature range from 0 to 100 degrees C are reported. Similar data for carbon tetrachloride up to 1118 bars and at temperatures from 25 to 54 degrees C have also been obtained. The data have been fitted to various empirical and theoretical expressions involving relative permittivity, making use, where necessary, of densities determined by other workers.