Mixture-casting Nafion(R) with quaternary ammonium bromides has been used to increase the mass transport of large redox species that are unable to be detected at a Nafion(R) modified electrode. However, the mixture-cast membranes lose selectivity against anions. In an attempt to increase selectivity, this study examined the effects of annealing the mixture-cast membranes on the mass transport, ion-exchange capacity, and selectivity of the membranes. Cyclic voltammetry, fluorescence microscopy, and ion-exchange capacity measurements were employed to examine the mass transport and selectivity of the membranes. The selectivity against anions was increased by annealing the mixture-cast membranes. Superselectivity was obtained by annealing recast, mixture-cast membranes containing tetrapropylammonium, tetrabutylammonium, and tetrapentylammonium salts. The recasting process allowed for the extraction of excess bromide salts from the membranes and formed membranes that were more reproducible and mechanically stable.