Thermodynamic properties of pentacyclo[5.4.0.0(2,6).0(3,10).0(5,9)undecane have been determined. The heat capacity was measured by vacuum adiabatic calorimetry (T = 5.0 K to T = 320.6 K) and by the triple heat-bridge method (T = 300 K to T = 480 K). One solid-to-solid transition was discovered at T = 164.4 K with molar enthalpy of transition : Delta(trs)H(m)(o) = (4.861 +/- 0.038) kJ . mol(-1). The fusion temperature is T = 475.8 K, and the molar enthalpy of fusion is Delta(fus)H(m)(o) = (6.38 +/- 0.12) kJ . mol(-1). Standard molar thermodynamic functions of the crystal obtained from the experimental heat capacity at T = 298.15 K are C-sat,C-m = (184.41 +/- 0.7)J . K-1. mol(-1); Delta(O)(T)S(m)(o) = (212.1 +/- 0.9)J . K-1. mol(-1); and Phi(m)(o) = Delta(O)(T)S(m)(o) - Delta(O)(T)H(m)(o)/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 range T = 164.4 K to T = 475.8 K. The entropy of the solid-to-solid transition at T = 164.4 K 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 : Delta(sub)H(m)(o)(336.86 K) = (54.71 +/- 0.94) kJ . mol(-1). The vapor pressure, measured by means of the integral effusion Knudsen method in the range T = 273.21 K to T = 323.40 K, may be expressed by the equation : In(p/Pa) = (25.74 +/- 0.44) - (6598 +/- 131) .(K/T). The weight-averaged value of the molar sublimation enthalpy : Delta(sub)H(m)(o)(298.15 K) = (55.85 +/- 1.00) kJ . mol(-1), was obtained using Delta(cr)(g)C(p,m) = 49 J . K-1. mol(-1). The third-law standard molar entropy at the pressure p = 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 : Delta(c)u degrees = - (43332.2 +/- 31.3) J . g(-1), and the molar enthalpy of combustion : Delta(c)H(m)(o) = -(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 : Delta(f)H(m)(o)(cr,298.15 K) = (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 Delta(f)H(m)(o)(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 at T = 298.15 K : Delta(O)(T)S(m)(o)(T,g) = 330.77 J . K-1. mol(-1), is in satisfactory agreement with the experimental value.