Negative ion photoelectron (NIPE) spectra of the radical anion of cyclopropane-1,2,3-trione, (CO)(3)(center dot-), have been obtained at 20 K, using both 355 and 266 nm lasers for electron photodetachment. The spectra show broadened bands, due to the short lifetimes of both the singlet and triplet states of neutral (CO)(3) and, to a lesser extent, to the vibrational progressions that accompany the photodetaclunent process. The smaller intensity of the band with the lower electron binding energy suggests that the singlet is the ground state of (CO)(3). From the NIPE spectra, the electron affinity (EA) and the singlet-triplet energy gap of (CO)(3) are estimated to be, respectively, EA = 3.1 +/- 0.1 eV and Delta E-ST = -14 +/- 3 kcal/mol. High-level, (U)CCSD(T)/aug-cc-pVQZ// (U)CCSD(T)/aug-cc-pVTZ, calculations give EA = 3.04 eV for the (1)A(1)' ground state of (CO)(3) and Delta E-ST = -13.8 kcal/mol for the energy gap between the (1)A(1)' and (3)A(2) states, in excellent agreement with values from the NIPE spectra. In addition, simulations of the vibrational structures for formation of these states of (CO)(3) from the (2)A(2)" state of (CO)(3)(center dot-) provide a good fit to the shapes of broad bands in the 266 nm NIPE spectrum. The NIPE spectrum of (CO)(3)(center dot-) and the analysis of the spectrum by high-quality electronic structure calculations demonstrate that NIPES can not only access and provide information about transition structures but NIPES can also access and provide information about hilltops on potential energy surfaces.