AbstractThe local equilibrium assumption commonly employed for the transport through the interface of a charged membrane has been analysed from a simplified electric double layer model. This layer is characterized on the basis of a surface potential arising from a non‐zero surface charge density placed on the membrane surface. The dependence of deviations from local equilibrium on the characteristic parameters of the problem is shown. Connection with the classical treatment by Donnan is discussed. Although the complexity of the problem calls for a number of simplifications, the results obtained appears to be significative. Thus, the analysis carried out displays not only that deviations from equilibrium are small, but also that in the limit of small fluxes, omission of a detailed study of the electric double layer is probably justified when characterizing ionic transport through common synthetic membran