AbstractThe electrodialysis of monocarboxylic acids and their potassium salts in the series formic acid to valeric acid has been investigated. A laboratory‐scale continuous‐flow unit was operated over the current density range 0·4 to 6·0 mA/cm2with total feed concentrations of 13·1 and 26·2 meq/1. Initial experiments were conducted with chloride and sulphate, for which the total transport and selectivities are established. At pH values set to assure 95% dissociation of the monocarboxylic acids, the transport (in the presence of chloride) of formate, acetate and propionate was about 20%, but that of butyrate and valerate was less. Acid salt selectivities (transport compared with chloride) ranged from 0·5 for formate through propionate to 0·3 for butyrate and valerate.For butyrate, the percentage transport increased approximately linearly with current density and was a random function of equivalent fraction of butyrate in the feed. Increases in total concentration produced no increases in transport. Butyrate selectivities were somewhat independent of current density and of equivalent fraction.Throughout the study, there was only limited evidence of fouling of the membrane by the organic acids and their salts, although valerate did produce temporar