The structure of HTiNbO 5 at 10 and 300 K and of DTiNbO 5 at 300 K has been investigated by neutron powder diffraction. The TiNbO 5 framework and the octahedral distortions previously found for the alkalititanoniobates by X-ray diffraction are confirmed. The position of hydrogen (or deuterium) has been determined: very strong OH … O hydrogen bonds with O … O distances ranging from 2.51 to 2.63 Å are observed. The similarity of these compounds with acids is shown, explaining their ion exchange properties. The absence of protonic conductivity can be explained from structural arguments. The open character of this structure, which can play a part in intercalation and deintercalation reactions is also discussed.