The transport of selected inorganic acids (HCl, H3PO4) through an anion-exchange membrane, Neosepta-AFN, was investigated in a two-compartment counter-current continuous dialyzer with single passes at steady state. The basic data obtained were completed by the measurement of sorption isotherms. The mass transfer rate of the individual acids was quantified by the diffusivity of acid in the Membrane, which was determined from the acid concentrations in the streams entering and leaving the dialyzer. For this purpose, two ordinary differential equations describing the steady-state concentration profiles of acid in both the compartments of the dialyzer were numerically solved in connection with an optimizing procedure. In the mathematical model used, the mass transfer resistances in the liquid films on both sides of the membrane and the flux of solution through the membrane were taken into account. All the experiments, carried out at a constant temperature of 25 degrees C with subsequent data treatment, revealed that the diffusivity of hydrochloric acid is about one order of magnitude higher than that of phosphoric acid. Moreover, regarding the dependence of the diffusivity of hydrochloric acid upon its concentration in the membrane, a maximum at an acid concentration of 0.622 kmol/m(3) can be identified, while the diffusivity of phosphoric acid remains practically constant.