The CBS-QB3, CBS-APNO and Gaussian-3 model chemistries have been used to determine the ionic and neutral heats of formation and the adiabatic ionization energies ( IEa) derived therefrom, for the ca 30 principal isomers of the C3H2O•+ and the C4H4O•+ families of radical cations. Theory and experiment are in excellent agreement for those molecules whose experimental IEa has been accurately measured. In contrast, large deviations from the computed values were found for a great many ionic heats of formation reported in the literature. These deviations largely arise from the uncertainty in the heat of formation of the corresponding neutral species for which often only a rough estimate is available. A useful by-product of this study is that it permits the evaluation of new Benson-type group additivity ( GA) terms appropriate for highly unsaturated oxygen containing molecules. Several new GA terms are proposed but it is also argued that a single GA term for the ketene function cannot be defined.