Acetic acid/sodium acetate and propionic acid/sodium propionate were separated by the diffusion dialysis technique using Neosepta AFN-7 and Selemion DSV membranes. Accounting for molarities of carboxylic acids and salts in broths produced by the pH controlled bacterial fermentation (pH 4-6), the experiments were carried out with carboxylates in excess of acids. The sorption equilibria established for acetic acid, propionic acid, and the sodium salts of both acids (single solute experiments) revealed a high sorption of acids in both membranes and the rejection of carboxylates. The partition coefficients were found to be from unity up to 2 for the acids and 0.04-0.05 for the salts. Reflecting a high sorption, the fluxes for the acids amounted from 1.5 to 2.0 mol. m(-2) . h(-1) and only 0.07-0.08 mol. m(-2) . h(-1) for the salts (one molar solutions). The separation factors computed upon the experiments performed with the ternary solutions were found between 20 to 37 for the Neosepta AFN-7 membrane and about 29 for the Selemion DSV membrane. Accounting additionally, for the results of the separation of lactic acid from sodium lactate reported in our earlier paper the results prove the diffusion dialysis to be applicable to the separation of mean strength and weak carboxylic acids from their salts.