Differences in equilibrium properties of hydrogen- and deuterium-bonded systems are considered. The potential energy surface is assumed to be independent of the isotopic species and the differences attributed to vibrational motion. A useful understanding of the problem can be gained by considering a hydrogen-bonded species as if it were a linear triatomic molecule AH … B with three significant vibrational modes—the A—H stretching mode and the doubly degenerate H-bond bend. Differences in the stabilities of H- and D-bonded species are related to differences in the zero-point energies of these vibrations.