The structure of pyrochlore-type antimonic acid, HSbO3 . 0.5H(2)O, has been investigated using powder diffraction of neutrons. The structure (space group ) contains an [Sb2O62-] framework, with interstitial oxygens present as a mixture of H2O and H3O+. Three H-atom sites have been identified, (1) close to the [Sb2O62-] framework in hydroxyl groups (Sb-OH), (2)H in water molecules H-bonded to framework hydroxyls, and (3)H in H3O+ ions H-bonded to O-atoms of the intercavity windows. An intraframework equilibrium of the type Sb-OH + H2O reversible arrow Sb-O- + H3O+ provides the mechanism for intracavity chemical exchange and proton conduction.