The unimolecular dissociation of hydrogen squarate and the squarate radical anion has been studied by electrospray ionization mass spectrometry (including collisionally induced dissociation) and quantum chemical calculations, providing consistent reaction models. In both cases, consecutive decarbonylations are observed as the dominating fragmentations. The reverse of these reactions corresponds to the successive cyclooligomerization of CO, which constitutes the most atom-efficient route to the cyclic oxocarbons. The reaction models indicate moderate barriers for CO addition to HCnOn- and CnOn•-, respectively, being larger for the former than for the latter. Cyclooligomerization leading to a neutral product is endothermic, while the analogous one-electron reductive coupling is exothermic. The analysis shows that the addition of an electron is essential for cyclooligomerization to give the cyclic four-CO squarate structure.