The mechanism of anion formation from core-excited carbon dioxide is investigated in the C 1s-excitation regime (280-340 eV), where negative-ion-positive-ion coincidences and negative-ion-positive-ion-positive-ion coincidences experiments are performed. O- formation occurs efficiently upon C 1s-->pi(*)-excitation (290.7 eV) from the singly charged cation CO2+. This anion is measured in coincidence with the singly charged atoms C+ and O+. The formation of atomic cations is accompanied by a substantial kinetic energy release. This suggests that an intermediate CO++ is formed together with O-, where the dication decays via fission. The results are discussed in terms of an anion formation mechanism from core-excited molecules considering previous experimental results. (C) 2004 American Institute of Physics.