A fundamental question about the hydrogen bond is whether the hydrogen is located in the middle of the two electronegative atoms, in a single-well potential, or else is closer to one of them:and jumping between them, in a double-well potential. This question has been of interest recently because short, strong hydrogen bonds have been proposed to provide stabilization In some enzyme-catalyzed reactions. The NMR method of isotopic perturbation of equilibrium is now used to get an unambiguous answer for the intramolecular hydrogen bond of the enol of 2-phenylmalonaldehyde-alpha-d in CDCl3 and pyridine-d(5). The equilibrium isotope shift, which is large, downfield, and dependent on temperature, was measured in both H-1 ana C-13 NMR spectroscopy. This result shows that the intramolecular hydrogen bond of 2-phenylmalonaldehyde enol is asymmetric, corresponding to the:presence of two equilibrating tautomers.