The heat capacities of SeF6 and WF6 were measured with an adiabatic calorimeter in the temperature range between 6 and 290 K. SeF6 and WF6 transformed from the crystalline form to the plastically crystalline and finally to the liquid form. Their temperatures and entropies are as follows : SeF6, T-trs = 134.50 +/- 0.02 K, Delta(trs)S = 28.12 +/- 0.01 J K-1 mol(-1), T-fus = 239.24 +/- 0.01 K, Delta(fus)S = 21.87 +/- 0.01 J K-1 mol(-1) WF6, T-trs = 264.95 +/- 0.02 K, Delta(trs)S = 36.50 +/- 0.01 J K-1 mol(-1), T-fus = 275.00 +/- 0.02 K, Delta(fus)S = 14.88 +/- 0.01 1 K-1 mol(-1). The sublimation enthalpy of SeF6 was also measured at 205.10 K. The molar enthalpy and entropy of sublimation at 205.10 K were 24.96 +/- 0.04 kJ mol(-1) and 121.7 +/- 0.2 J K-1 mol(-1), respectively. The third-law entropy of SeF6 at 298.15 K and 10(5) Pa was calculated from the present results to be 312.9 +/- 0.5 J K-1 mol(-1). This value agrees well with the statistical value (313.6 +/- 0.4 J K-1 mol(-1)) calculated from the spectroscopic data, indicating that SeF6 becomes completely ordered at 0 K. The transition entropies of SF6, SeF6, and WF6 were reproduced by a model considering the entropy associated with the excited levels of the rotational vibration. It was found that the large transition entropy of the MF(6) family is due to the broad potential surface caused by the orientational frustration inherent to the bcc structure of the high-temperature phase.