The B3LYP/6-31G(d) methodology was applied to the study of the diketone pentacy-clo[5.4.0.02.6.03.10.05.9] undecane-8,11-dione (1), the hydrocarbon pentacyclo[5.4.0.02.6.03.10.05.9]undecane (2) and the ketone pentacyclo[5.4.0.02.6.03.10.05.9] undecane-8-one (3). Also examined were the oxygen-protonated species 1H+ and 3H+. The diketone pentacyclo[5.4.0.02.6.03.10.05.9] undecane-4,8-dione (4) and the ketone pentacy- clo[5.4.0.02.6.03.10.05.9]undecane-4-one (5) and their protonated forms were used for comparison purposes. This information was used to construct several isodesmic reactions, allowing the quantitative estimate of the influence of the carbonyl groups on the stability of the various species in the gas phase. These results were in excellent agreement with the experimental thermochemical and structural data available. Analyses by means of the natural bond orbital theory and simple electrostatic models were also performed. © 2001 John Wiley & Sons, Ltd.