A radio frequency reentrant resonator has been used to determine dielectric constants for one methane + propane mixture and two methane + propane + hexane mixtures along isotherms between 293 K and 313 K, at pressures to 10 MPa. These measurements were made simultaneously with density measurements on the same mixtures, allowing molar polarizabilities to be calculated. Changes in polarizability on mixing were estimated, with component values from literature sources, and were found to average less than 0.6% of mixture polarizability values.