The influence of the electrolyte on the cation exchange process of Nafion membranes has been studied using 2-(2′-pyridyl)benzimidazole (2PBI) as a fluorescent probe. The relative fluorescence intensities at 360 and 470 nm of this molecule are found to be different in Na⁺-exchanged Nafion membranes. The relative contributions of the two phototautomers emitting at these two wavelengths is found to depend on the method of incorporation of Na⁺ ion as well as the fluorophore. The fluorescence data, in conjunction with the lifetimes, are used to develop a model to rationalize the role of co-ions present in the electrolyte on the ion exchange process of Nafion.