A technical device, designed to measure directly the membrane permeability in non-equilibrium steady state, has been developed for the study of membrane potential differences. It is shown that the membrane potential does not depend on membrane nature and bulk concentrations only, but also on local concentrations on membrane walls due to experimental mixing conditions. This observation leads us to define a specific contribution of membrane ΔΦ sm : ΔΦ mb = ΔΦ lj + ΔΦ sm where ΔΦ mb is the measured membrane potential and ΔΦ lj is the liquid junction potential. ΔΦ sm includes two terms: • a diffusion term depending on concentration ratio, and • a charge term depending also on the concentration level. From theoretical and experimental treatment, we can show that our artificial albumin membranes are negatively charged. We can also determine local ionic transference numbers inside the membrane phasis. The studied electrolytes are NaCl or CaCl 2. We observe that the relative mobility of Ca 2+ decreases inside membrane phase whereas the relative mobility of Na + increases with respect to the relative mobility of Cl −.