The infrared spectra of aqueous O-phospho-l-serine provides evidence for the existence of intermolecular hydrogen bonding at the ionized carboxylate group. In addition, a van't Hoff plot gives an enthalpy of - 2.8 kcal/mol for the bond energy of the equilibrium between the two forms of the carboxylate, a value well within the normal range of hydrogen bond energies. The addition of equimolar Ca2+ to O-phospho-l-serine does not substantially disrupt the hydrogen bonding of the carboxylate. The data presented here support the recent suggestion that anionic lipid headgroups may be the site of a proton-conducting pathway (Haines T.H. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 160-164). © 1984.