Osmotic coefficients are determined for tetra-n-butylammonium perchorate (TBAClO(4)) in acetonitrile. Filter equations are fit to the data, and the resulting fitting parameters are used to calculate activity coefficients for TBAClO(4). The activity coefficients are used to control electrolyte activities so that the junction potential across a permselective anion exchanger is near zero. An electrochemical cell containing this junction is used to examine the influence of electrolyte activity on the formal potentials measured for ferrocene/ferrocenium and decamethylferrocene/decamethylferrocenium. It is clear that the oxidized forms of both redox couples are ion-paired with perchlorate anions in the supporting electrolyte and that this ion-pairing has a significant influence on the formal potentials.