The relative permittivities (epsilon(r)) of trifluoromethyl methyl ether (HFE 143a) and pentafluoroethyl methyl ether (HFE 245mc) were measured as a function of temperature and pressure: (303 to 383) K and (1.5 to 31.6) MPa for HFE 143a and (313 to 323) K and (1.5 to 28.4) MPa for HFE 245mc. The relative permittivity was fitted to the reduced density (rho(r)) using the function (epsilon(r) - 1)/(2 epsilon(r) + 1). The apparent dipole moment (mu*) of the liquid phase was calculated for each fluid using Kirkwood's theory of molecular polarizability.