The paper deals with the diffusion dialysis of hydrochloric acid in a mixed batch cell with an anion-exchange membrane NEOSEPTA-AFN. The systems HCl/membrane/H2O and HCl-FeCl3/membrane/H2O were investigated. The overall dialysis coefficient and the membrane mass transfer coefficient were used for a quantitative evaluation of the mass transfer rate of Cl- ions in the membrane. Both these quantities were calculated by numerical integration of differential equations, which describe the unsteady state transport of Cl- ions, with subsequent optimization procedure. The model suggested presumes the zero flux of Fe3+ ions, existing equilibria and mass transfer resistances on both sides of the membrane. It was found that, in the range of HCl concentration from 0.27 to 3.48 kmol m(-3), and of Fe3+ ions from 0.17 to 1.66 kmol m(-3), the rate of transport of Cl- ions decreases with increasing concentration of Fe3+ ions in the solution dialyzed.