The objective of this work was to demonstrate that the efficiency of iontophoretic transport across the skin (which is measured in terms of an ion's transport number), either for drug delivery or for therapeutic drug monitoring, depends implicitly on the molar fraction of the species of interest over a wide range of experimental conditions both in vitro and in vivo. Three sets of data from the literature were assessed to establish the direct relationship between transport number and mole fraction. Linear regression between these parameters yielded slopes which correlated with the charge-carrying efficiency of the ion considered. The latter, furthermore, was proportional to the corresponding aqueous mobility and to the transport number of the ion when it is the sole species available for migration from its electrode solution (the so-called “single-carrier” situation). Finally, the principles illustrated here were equally applicable to in vitro experiments and to in vivo data obtained in a clinically relevant study (specifically, the reverse iontophoretic monitoring of lithiemia in bipolar patients). Not only does this validate an in vitro model typically used in iontophoresis research, it also demonstrates the potential of this approach to predict the feasibility of iontophoretic transport across the skin.