The dipolarity/polarizability parameter pi*, introduced by Kamlet and Taft, was measured for difluoromethane (HFC 32) and pentafluoroethane (HFC 125) and compared with those previously reported for 1,1,1,2-tetrafluoroethane (HFC 134a). This was carried out as a function of temperature and pressure, to cover the liquid and supercritical states over the range 30-130 degrees C and 40-300 bar. The change in dipolarity/polarizability with the reduced density is explained in terms of local density augmentation and a van der Waals model is used to predict the changes in solvent properties. The mean sphere approximation (MSA) model was applied to the data and found to produce an excellent fit for all of the solvents in both the liquid and supercritical states.