H-1 and P-31 NMR spectroscopic measurements were used to examine the behavior of H3PO4 sorbed in Nafion(R) 117 membranes. The P-31 NMR signal for phosphoric acid sorbed in Nafion was found to have the same chemical shift as that of 100% phosphoric acid in bulk liquid phase. This, however, was broadened compared to that in the bulk acid (FWHM = 121 Hz versus 8 Hz for bulk 100% H3PO4), suggesting lower mobility, possibly due to hydrogen-bonding-type interactions. In the H-1-NMR spectrum for H3PO4-equilibrated Nafion, the peak was shifted downfield compared to a relatively dry membrane (similar to 2 H2O per -SO3H), consistent with a higher mole fraction of acidic protons. The presence of a single peak also indicates an exchange of protons between the sulfonic acid groups, phosphoric acid and residual water. The latter may in fact mediate the exchange between -SO3H and H3PO4. The peak was relatively broad (FWHM = 240 Hz) compared to those in "dry" (FWHM = 100 Hz) and water-equilibrated Nafion (FWHM = 80 Hz), indicating lower mobility and/or the effect of finite proton exchange rates.