The chronopotentiometric response of a cation exchange membrane in contact with a symmetrical 0.01 M NaCl solution was investigated as a function of the applied current density below the limiting one. Taking into account classical assumptions such as the variation of concentrations associated to the Teorell-Meyer-Sievers (TMS) segmentation model, the concept of the Nernst layer model, the quasi-electroneutrality condition within the diffusion boundary layers (DBL) and the validity of the Donnan equation at the solution-membrane boundaries, theoretical values of the transmembrane potential difference were obtained which fit very well the experimental V-t responses. Moreover, this method allows the calculation of the DBL thickness. In addition, a decrease of the DBL thickness was observed when the current density increases. This result was attributed to the occurrence of convection effects inside the DBL.