Is a hydrogen bond symmetric (single-well potential) or asymmetric (double-well potential)? The NMR method of isotopic perturbation of equilibrium was used to answer this question for the monoanions of a wide variety of O-18-labeled dicarboxylic acids. The observed O-18-induced isotope shifts, especially at the ipso carbons, demonstrate that these exist as a pair of equilibrating tautomers in both aqueous and organic solvents. This conclusion for organic solvents is opposite a previous one based on similar data. As a further test, primary isotope shifts, which had been diagnostic for a single-well potential, were reinvestigated. The monoanions of 1,2-cyclopentenedicarboxylic, 3,4-furandicarboxylic, and 3,4,5,6-tetrahydrophthalic acids have negative primary isotope shifts and are confirmed as having asymmetric hydrogen bonds. Although hydrogen phthalate has a positive primary isotope shift, it too is judged to have an asymmetric hydrogen bond, according to O-18-induced isotope shifts, which are considered more reliable.