The enhancement in ion selectivity of a cation exchange membrane by amine modification of its surface has been assessed. The amines used were butylamine (BA) and two forms of diamine. The precursor membrane supplied was Nation 115 with sulphonyl fluoride (SOZ(2)F) side groups, which was then hydrolysed into the ion exchange form containing sulphonate (SO3-K+) functional groups that attract water, which in turn carries the ions. In order to reduce the ionic nature of the membrane, the precursor was modified with amines; the resulting aminated groups are not ionised on hydrolysis. The influence of the surface modification on the selectivity and the resistivity of the membrane was investigated to reveal that amination in some cases was detrimental to the selectivity/resistivity balance. The diamines gave better performance, perhaps due to the incorporation of cross-links. (C) 2004 Elsevier B.V. All rights reserved.