The standard molar energy of combustion of pentacyclo[5.4.0.02,6.03,10.0 5,9]undecane-8,11-dione has been determined by using a static bomb calorimeter. The vapour pressure of the compound was measured in the interval T = 325.64 K to T = 340.97 K by the Knudsen-effusion technique. Enthalpy of fusion and enthalpy of transition measurements were carried out by differential scanning calorimetry (d.s.c.). From the experimental results of this work, the standard enthalpies of combustion, sublimation, and formation in the crystalline and gaseous state at the temperature 298.15 K have been derived for pentacyclo[5.4.0.02,6.03,10.0 5,9]undecane-8,11-dione: ΔfHom (cr) = -(206.2 ± 3.2) kJ·mol-1, ΔsubHom = (92.6±1) kJ·mol-1, ΔfHom (g) = -(113.6±3.4) kJ·mol-1. Strain energy due to the presence of the two carbonyl groups in the molecule is estimated by three different methods with consistent results. © 1999 Academic Press.