The competitive absorption of salt mixtures containing tetramethylammonium (TMA) and/or tetraethylammonium (TEA) cations, respectively, into a Nafion(TM) 117 cation-exchange membrane, under equilibrium conditions, was measured experimentally and modelled theoretically. Uptake experiments were performed, with Nation equilibrated in an aqueous 0.1 M nitrate salt solution containing either: (a) one of the two quaternary ammonium cations and Li+, Na+, or Cs+ or (2) a mixture of TEA and TMA. Equilibrium absorption of the various two-component salt solutions in Nafion was modelled using the molecular-level partition coefficient theory developed by Bontha and Pintauro [Chem. Eng. Sci. 49 (1994) 3835]. Without the use of adjustable parameters, the model predicted accurately cation concentrations for all salt systems except Cs+/TMA. The observed and computed absorption selectivity ordering in Nafion was (CH3CH2)(4)N+ > (CH3)(4)N+ > Cs+ > Na+ > Li+. The failure of the model to predict cation uptake for Cs+/TMA mixtures was attributed to small errors in the computation of an ion hydration parameter for TMA, coupled with the fact that the Cs+ and TMA cations were nearly the same size. (C) 2003 Elsevier Science B.V. All rights reserved.