In 1941 Ketelaar1 published accurate measurements on the infra-red absorption and reflexion spectra of potassium hydrogen fluoride (KHF2). The two strong peaks of 1450 and 1222 cm.−1 were interpreted as a doubling of the fundamental asymmetric valence frequency v3 belonging to the vibration of a proton between two F− ions. The doubling was assumed to be caused by a double-minimum potential curve of the type occurring in the NH3 molecule. The splitting of the lowest vibrational level was estimated to be 25 cm.−1 and the distance between the two equilibrium positions 0.70 A.