The energies of vertical electron attachment to cyclobutanone, its 2- and 3-oxa derivatives, 1,2-cyclobutanedione, and 2,2',4,4'-tetramethyl-1,3-cyclobutanedione have been measured in the 0-6 eV energy range by means of electron transmission spectroscopy. The energies of temporary anion formation are nicely reproduced by shifting and scaling the virtual pi* orbital energies calculated for the MP2/6-31G*-optimized geometries of the neutral molecules. The linear correlation between experimental and calculated energy values reported in the literature for the empty pi*(CC) orbitals fits the present data on the pi*(CO) orbitals with the same accuracy. The two pi* anion states observed in the ET spectrum of the 1,3-dione (split by 1.25 eV) are assigned to the in-phase and out-of-phase combinations in order of increasing energy, i.e., the sequence dictated by through-space interaction. The calculations indicate a large through-space splitting (>2 eV) of the empty carbonyl pi* orbitals, reduced by through-bond destabilization of the in-phase combination.