The acidity of niobium pentoxide calcined at 573 K has been studied by interacting water molecules and the solid acid using H-1 broad-line NMR at 4 K and MAS NMR at room temperature. H-1 Broad-line NMR in ’rigid lattice’ conditions allows the determination of the numbers of the oxygen-protonated species (H3O+ and H2O ... HO species, unreacted OH groups or excess H2O molecules) present when water molecules used as base react with a Bronsted acid. With niobic acid, the method showed a continuous increase in the hydronium ion concentration with the number of water molecules. The ionization coefficient, determined when the ratio of the number of water molecules to that of the acidic hydroxyl groups of niobic acid is one, is 0.2. This value is similar to the ionization coefficients found for some HY zeolites.