Density functional and coupled cluster calculations on neutral monodehydro[12]annulenes (C(12)H(10)) reveal a global minimum that should be kinetically stable. At the CCSD(T)/cc-pVDZ//BHLYP/6-31G* level, the unsymmetrical CTCTC conformer 1a lies at least 3 kcal/mol below all other isomers studied. The two isomers closest in energy to 1a are Möbius structure 5a (CCTCC) and all-cis 6a. Isomer 1a can undergo conformational automerization with E(a) = 3.9 kcal/mol, implying that this process would be rapid on the NMR time scale, and computed (1)H NMR parameters (GIAO-B3LYP/6-311+G**//RHF/6-31G*) are presented. Cumulenic dehydro[12]annulene isomers, with 1,2,3-butatriene subunits, were found to be reactive intermediates in the interconversion of different configurations of the alkyne forms. Pathways for configuration change of 1a, and for subsequent rearrangement to biphenyl, were investigated. The 28 kcal/mol overall barrier for the lowest energy pathway connecting 1a to biphenyl suggests that 1a is kinetically stable with respect to valence isomerization.