This research examined how the physical properties of Nafion® are affected by mixture casting Nafion® with quaternary phosphonium salts. Quaternary phosphonium salts can be mixture cast with commercially available Nafion® suspension to form membranes with decreased proton exchange capacity and altered mass transport of cations. anions and neutral species through the membrane. Since quaternary phosphonium salts are similar in size and hydrophobicity to quaternary ammonium salts, the mass transport of cations, anions and neutral species through tetraethylphosphonium bromide and tetrabutylphosphonium treated Nafion® is similar to tetraethylammonium bromide and tetrabutylammonium treated Nafion®, respectively. Although there is no statistically significant difference between transport through tetrabutylphosphonium bromide treated Nafion® membranes and tetrabutylammonium bromide treated Nation® membranes, there is a detectable difference for the transport of ferricyanide, methyl viologen and tetramethylphenylenediamine through tetraethylphosphonium bromide treated Nafion® membranes. The difference is consistent with the fact that tetraalkylphosphonium bromides are slightly larger than tetraalkylammonium bromides, so the resulting pore size for a tetraalkylphosphonium bromide treated Nafion® membrane is larger than the pore size for a tetraalkylammonium bromide treated Nafion® membrane. The proton exchange capacity of tetraalkylphosphonium bromide treated membranes is larger than tetraalkylammonium bromide treated membranes. This data is expected since phosphonium salts are less hydrophobic than the ammonium salts and will have a lower affinity for the sulfonic acid exchange sites. Overall, this research indicates that quaternary phosphonium bromide salts can be employed to further tailor the physical and chemical properties of Nafion® membranes. © 2005 Elsevier B.V. All rights reserved.