Glassy carbon electrodes coated with the poly(estersulfonate) Kodak AQ55 were used in acetonitrile solutions of various supporting electrolytes to study the ion-exchange voltammetric behaviour of some electroactive cations, namely the monocharged cation (ferrocenylmethyl)trimethylammonium (FA(+)) and the dications Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine), methylviologen, butylviologen and heptylviologen. For all the electroactive cations, it was found that the success in their ion-exchange is determined by the size of the supporting electrolyte cation. In particular, lowering the steric hindrance of the supporting electrolyte cation results in a more significant ion-exchange competition with the electroactive cation. As far as different redox analytes are concerned, it was observed that the preconcentration efficiency increases with increasing charge and for electroactive species with the same ionic charge, parallels the order of decreasing steric hindrance. In the case of the viologens, this last factor seems to overcome the hydrophobic one, where the small (but less hydrophobic) methylviologen is preconcentrated more efficiently than the large (but more hydrophobic) heptylviologen.