Polymeric cation exchange membranes (PEM) are used in fuel cell technology. These membranes act like a physical barrier between the anode and cathode but the diffusion through the membrane should allow the transport of protons from the anode to cathode at a rate that is enough to supply the energy requirements. They avoid the direct reaction of oxygen and hydrogen that would diminish fuel cell efficiency.The membrane has to be conditioned before use. This conditioning step changes the membrane counter-ion and modifies its water content which affects the diffusion coefficients. Diffusion experiments were carried out in a reactor with two semi-cells of 250 mL with the membrane placed between the semi-cells. Diffusion through the membrane was studied using two system configurations: without applying any potential and applying zero potential drop across the membrane using two Ag/AgCl electrodes.Diffusion of chloride through the membrane is related to membrane permselectivity. Diffusion of hydrogen chloride was followed by measuring the pH of the diluted dissolution and the potential drop across the membrane with two Ag/AgCl reference electrodes. Several experiments were carried out using initial concentrations between 10-3 M to 0.1 M. Results were modelled using Nernst–Planck equations. Results obtained show that the diffusion of a co-ion is a very slow process and that the conditioning step enhances proton transport. Diffusion coefficients obtained were in the range of 10-11 cm2/s for chloride and 10-7 for proton.