The heat capacities of the crystalline, plastic crystalline, and liquid phases of trans- and cis-perfluorobicyclo(4,3,0)nonanes were measured by vacuum adiabatic calorimetry over the temperature range 5 K to 350 K and 5 K to 354 K, respectively. Two solid-to-solid phase transitions for the cis-isomer at T-trs (III to II) = 200.64 K and T-trs (II to I) = 245.63 K and one transition for the trans-isomer at T-trs (II to I) = 236.63 K were found. The triple point temperatures of the compounds are T-tp = 291.27 K and T = 248.05 K, respectively. The enthalpies and entropies of transition and fusion and the main thermodynamic functions have been obtained. A conclusion was drawn, on the basis of large values of solid-to-solid transition entropies and small values of fusion entropies, that both substances form plastic crystals. The thermodynamic properties of some bicyclic perfluorocarbons were compared with corresponding values for their hydrogen-containing analogs,