Ab initio and DFT calculations indicate that the charge-localized forms (C 2 v symmetry) of croconate (C 5 O 5 2 - ) and rhodizonate (C 6 O 6 2 - ) dianions cannot exist as a single entity. Instead, this form of the dianions can exist in the presence of counter cation(s) or in hydrogen-bonded networks. For the charge-delocalized (D 5 h symmetry) form of C 5 O 5 2 - , it is found that the HF/3-21G(d) and CASSCF(n,m)/3-21G(d) levels of theory yield C-C and C-O bond lengths that are within the experimental range. However, for the D 6 h tautomer of C 6 O 6 2 - , all the theoretical methods that have been tried led to optimized C-C bond lengths that are slightly (about 0.02-0.06 A) too long.