In iontophoresis, an electric field across the skin induces electromigration of exogenously applied and endogenously present ions. The approach can be used to improve dramatically drug delivery by elimination of competing co-ions in the externally applied formulation, as this maximizes the fraction of current carried by (i.e., the transport number of) the drug. In this study, the dependence of the transport number on the nature of the ions present at the anode and cathode was examined using 12 different combinations of ions (4 cations × 3 anions). Cationic transport numbers (t°_c+) were a function of their inherent mobilities and depended upon those of the single anion at the cathode; however, t°_c+ was independent of the counter-ion concentration. The extensive data obtained agreed well, furthermore, with a theoretical framework previously developed for this so-called “single-ion” situation. In addition, there was a strong correlation between ionic transport numbers in the skin and those in aqueous solution, which are easily estimated from measurements of ionic mobility. It follows that it should be possible, from the framework developed here, to establish a predictive tool, combining theory with simple experiments, for the optimization of iontophoretic drug delivery.