N-14 NMR spectroscopy has been used to observe the behavior of nitrate co-ions in cation exchange membranes equilibrated with NH4NO3. The solubility of excess nitrate salts in Nafion membrane depends on the counterion used, increasing in the order Li+ < NH4+ < Na+ < K+ < Csi and Na+ < Mg2+ < Al3+. The N-14 quadrupolar splittings of the NH4+ and NO3- ions in Nafion are strongly dependent on the water content and on the macroscopic orientation of the membrane, with nitrate having the larger splitting in all samples. The immobilization of the ions in Nafion membrane was further investigated by measuring N-14 spin-lattice relaxation times. T-1 values are reduced by a factor of similar to 10 for nitrate and 50-100 for ammonium ions in Nafion compared to aqueous solution.