Nafion(117) membranes in contact with acidic solutions were characterized by small-angle X-ray scattering and by acid solution uptake measurements. The principle of Donnan equilibrium was used to obtain independent estimates of the extent of counterion condensation in the membranes from the three experiments. The surprising conclusion of our study is that a large fraction of the protons in Nafion are in the condensed form when the membrane is immersed in pure water. Estimates of the fraction of condensed protons range from 72 to 98%.