Thermodynamic properties of pentacyclo[5.4.0.02,6.03,10.05,9]undecane have been determined. The heat capacity was measured by vacuum adiabatic calorimetry (T= 5.0 K toT= 320.6 K) and by the triple heat-bridge method (T= 300 K toT= 480 K). One solid-to-solid transition was discovered atT= 164.4 K with molar enthalpy of transition: ΔtrsHm° = (4.861 ± 0.038)kJ·mol−1. The fusion temperature isT= 475.8 K, and the molar enthalpy of fusion is ΔfusHm° = (6.38 ± 0.12)kJ·mol−1. Standard molar thermodynamic functions of the crystal obtained from the experimental heat capacity atT= 298.15 K areCsat,m=(184.4 ± 0.7)J·mol−1; ΔT0Sm° = (212.1 ± 0.9)J·K−1·mol−1; and Φm° = ΔT0Sm°− ΔT0Hm°/T= (102.7 ± 0.4)J·K−1·mol−1. Comparison of the thermodynamic characteristics of fusion and solid-to-solid transition confirms the existence of a plastic crystalline state in the rangeT= 164.4 toT= 475.8 K. The entropy of the solid-to-solid transition atT= 164.4 is about 25 per cent of the free-rotation entropy of isolated molecules. The enthalpy of sublimation was measured with a heat-conduction differential microcalorimeter: ΔsubHm°(336.86 K) = (54.71 ± 0.94)kJ·mol−1. The vapor pressure, measured by means of the integral effusion Knudsen method in the rangeT= 273.21 K toT= 323.40 K, may be expressed by the equation: ln(p/Pa) = (25.74 ± 0.44) − (6598 ± 131)·(K/T). The weight-averaged value of the molar sublimation enthalpy: ΔsubHm°(298.15 K) = (55.85 ± 1.00)kJ·mol−1, was obtained using ΔgcrCp, m= 49 J·K−1·mol−1. The third-law standard molar entropy at the pressurep= 101 325 Pa is (333.6 ± 3.4)J·K−1·mol−1, based upon the weight-averaged value of the molar sublimation enthalpy. The massic energy of combustion in oxygen: Δcu° = −(43332.2 ± 31.3)J·g−1, and the molar enthalpy of combustion: ΔcHm° = −(6345.2 ± 4.8)kJ·mol−1, were determined from the results of five experiments. The standard molar enthalpy of formation in the crystalline state: ΔfHm°(cr,298.15K) = (15.8 ± 4.9)kJ·mol−1. The standard molar enthalpy of formation in the gas state (obtained by using the weight-averaged sublimation enthalpy) is ΔfHm°(g,298.15 K) = (71.7 ± 5.0) kJ·mol−1. It was shown that the total strain energy simply reflects the sum of the strain energies in the independent rings which comprise the compound. I.r. and Raman spectra were recorded, and a vibrational analysis was performed on the spectral quantities. The standard thermodynamic properties in the ideal-gas state were calculated in the temperature range 100 K to 1000 K. The calculated standard molar entropy atT= 298.15 K: ΔT0Sm°(T, g) = 330.77 J·K-1·mol-1, is in satisfactory agreement with the experimental value.