The quantitative study of the Bronsted acidity of niobic acid (Nb2O5 . xH(2)O) using broad-line H-1 NMR at 4 K has been performed by interacting niobic acid, pretreated at 573 K under vacuum, with water molecules. The number of oxyprotonated species (H3O+, and H2O ... HO species formed, unreacted acidic OH groups or excess H2O molecules) deduced from the simulations of the broad-line H-1 NMR spectra shows a continuous increase in the number of H3O+ species with adsorbed water molecules. This increase may be due to a classical dilution effect or to a synergistic interaction between Bronsted and Lewis acid sites. These results are compared with those of some HY zeolites with or without framework defects.