Membrane permeation characteristics of formic, acetic, propionic, and butyric acids were studied over a wide range of operating conditions in the electrodialysis. These mono-carboxylic acids permeated through an anion-exchange membrane, but did not permeate through a cation exchange membrane. The flux of each mono-carboxylic acid in the single-acid system was significantly affected by solution pH: The flux increased rapidly at pH about 2, after which was kept constant. In the experiments of binary, and ternary acids systems, the order of the flux was formic acid>acetic acid>propionic acid>butyric acid. All the experimental results were analyzed with a mathematical model Which considered dissociation equilibrium of mono-carboxylic acids, electroneutrality in the solution, ion-exchange between the solution and the ion-exchange membranes, and ionic flux in the ion-exchange membranes based on the Nernst-Planck's equation. The model successfully explained the permeation behavior of single, binary, and ternary mono-carboxylic acids systems.