Ion exchange membranes (IEMs) are used for selective transport of ions between two solutions. These solutions are often different in concentration or composition. The membrane resistance (R-M) is an important parameter affecting power consumption or power production in electrodialytic processes. In contrast to real applications, often R-M is determined while using a standard 0.5 M NaCI external solution. It is known that R-M increases with decreasing concentration. However, the detailed effect of a salinity gradient present over an IEM on R-M was not known, and is studied here using alternating and direct current. NaCl solution concentrations varied from 0.01 to 1.1 M. The results show that R-M is mainly determined by the lowest external concentration. R-M can be considered as two resistors in series i.e. a gel phase (concentration independent) and an ionic solution phase (concentration dependent). The membrane conductivity is limited by the conductivity of the ionic solution when the external concentration, c(ext)< 0.3 M. The membrane conductivity is limited by the conductivity of the gel phase when c(ext)> 0.3 M, then differences of R-M are small. A good approximation of experimentally determined R-M can be obtained. The internal ion concentration profile is a key factor in modeling Rm. (C) 2014 Elsevier B.V. All rights reserved.