The transport of phosphoric and hydrochloric acids through anion-exchange membrane Neosepta AFN in a counter-current continuous dialyzer has been investigated. The basic transport characteristics of membrane/solution systems - the overall dialysis coefficient and the permeability of the membrane - have been evaluated at steady state. The determination of these characteristics is based on a numerical integration of the set of differential equations describing the concentration profiles in both compartments of the dialyzer followed by an optimizing procedure. Using all experimental data obtained at various acid concentrations and volumetric flow rates, it has been found that the permeability of the membrane is strongly affected by acid concentration - for phosphoric acid it decreases with an increasing acid concentration and for hydrochloric acid it increases with an increasing acid concentration. Moreover, except for the lowest acid concentration used, the permeability for phosphoric acid is lower than that for hydrochloric acid - in the concentration range of acid from 0.2 to 3.0 kmol m(-3), the permeability of Neosepta-AFN membrane has been in limits from 1.4 x 10(-6) to 2.1 x 10(-7) ms(-1) for phosphoric acid and in limits from 6.4 x 10(-7) to 3.1 x 10(-6) ms(-1) for hydrochloric acid.