The symmetry of the hydrogen bond in hydrogen cyclohexene-1,2-dicarboxylate monoanion was determined in chloroform using the NMR method of isotopic perturbation. As the temperature decreases, the O-18-induced C-13 chemical-shift separations increase not only at carboxyl carbons but also at ipso (alkene) carbons. The magnitude of the ipso increase is consistent with an O-18 isotope effect on carboxylic acid acidity. Therefore it is concluded that this monoanion is a mixture of tautomers in rapid equilibrium, rather than a single symmetric structure in which a chemical-shift separation arises from coupling between a desynrunetrizing vibration and anharmonic isotope-dependent vibrations, which is expected to show the opposite temperature dependence.