Conditions have been developed for the surface modification of Nafion(TM) 117 cation-exchange membrane by conversion to the sulfonyl chloride form and coupling with secondary amines to form sulfonamide groups. Nafion 117 membranes modified with diethylamine (13), dibutylamine (14), morpholine (15), bis(2-methoxyethyl)amine (16), and 8-aza-2,5,11,14-tetraoxapentadecane (17), monoaza-12-crown-5 (18), monoaza-15-crown-5 (19), and monoaza-18-crown-6 (20) were prepared and their competitive, proton-coupled transport of alkali metal cations was compared with that of unmodified Nafion 117 membrane. Alkali metal cation permeation of membranes 13-16 was found to be poor. However, membranes 17-19 which were produced by coupling with an acyclic or cyclic polyether amine having more than three oxygen atoms gave efficient alkali metal cation permeation which was comparable with that for unmodified Nafion 117 membrane. The permeation selectivity order for these surface-modified Nafion 117 membranes was Cs+ > Rb+ > K+ > Na+ > Li+ which differs from the ordering of K+ > Rb+ > Cs+ > Na+ > Li+ observed for unmodified Nafion 117 membrane.