The permeation of acetic (AA), propionic (PA), lactic (LA), oxalic (OA), citric (CA), and tartaric (TA) acids through the bipolar ion-exchange membrane Neosepta BP-1 (Tokuyama Corp.) was studied. It was found that the fluxes (J, mol cm(-2) s(-1))and mass-transfer coefficients (k, cm s(-1)) increase in the following order: CA < OA < LA < TA < PA less than or equal to AA. The transport processes in the Neosepta BP-1 membrane are concentration-dependent and can be described phenomenologically using I-Fick's law for diffusion. The permeation phenomena correspond to the solution-diffusion model similarly as to the permeation of carboxylic acids through strongly acidic cation-exchange membranes. However, in competitive AA-PA transport experiments, typically for strongly basic membranes, the separation ability of the BP-1 membrane with a preference toward AA was observed. The selectivity coefficients alpha (AA)(PA) calculated as the ratio of the respective mass-transfer coefficients vary in the range from 1.31 +/- 0.2 to 2.1 +/- 0.6. These values depend on the feed composition and the system arrangement, which means that alpha (AA)(PA) is always higher for the system with the anion-exchange layer is in contact with a feed solution. Rather low fluxes of PA, AA, and other acids, as compared to some monopolar membranes (Neosepta AFN-7, Nafion-120, Flemion), are promising for the application of the bipolar membrane in an electrodialytic separation of carboxylic acids from their aqueous solutions or mixtures.