Broad-line H-1 NMR at 4 K shows that, when Nafion-H interacts with water molecules at low concentrations, the only species formed is the hydronium ion. The chemical shift of the unsolvated hydronium ion in Nafion-H with 1 H2O/SO3-, determined by H-1 MAS NMR at room temperature, is 10.4 ppm relative to external TMS.