A better understanding of ion-transport mechanisms requires separate information on the equilibrium (partitioning) and kinetic (diffusivity) properties of the membranes with respect to ions. This work presents a novel non-stationary-diffusion method to determine salt diffusion and partitioning coefficients under (quasi)-linear conditions of relatively small concentration differences. An ion-exchange membrane supported by a relatively thick coarse-porous support (glass frit) is placed in a two-compartment stirred cell. The salt concentration in one compartment is kept stationary during the measurement whereas in the other compartment, the initial solution is rapidly replaced by a solution of different concentration. As a result, there is a time-dependent electrical response due to a progressive redistribution of applied concentration difference between the membrane and the porous support and the different ion perm-selectivities of those media. A mathematical model is developed to interpret the data. The rate of signal relaxation is primarily controlled by the diffusion permeability of the membrane but is also affected by the salt partitioning. In addition, osmotic trans-membrane volume transfer has a significant impact on the relaxation process, so it needs to be taken into account. The osmotic permeability has been determined in separate measurements. Systematic studies have been carried out at various NaCl concentrations with Nafion 120 and type 10 Fujifilm ion-exchange membranes. The results obtained for Nafion 120 are in agreement with the literature data. Well-established techniques have also been used to validate the novel approach. This allows for a relatively simple determination of salt permeability and partitioning coefficient in addition to the ion perm-selectivity, which is the only parameter available from the conventional measurements of stationary membrane potential.