Bipolar membrane electroacidification (BMEA) technology has been applied to skim milk protein, in order to produce high purity casein isolates. BMEA uses a property of bipolar membrane to split water and the action of monopolar membranes for demineralization. When the H+ are generated at the bipolar membrane, one cationic charge inherent to the solution being acidified must cross the cation exchange membrane (CEM) to keep the solution electrically neutral. The purpose of this study was to monitor the migration of charge through the CEM to counterbalance H+ generated at the bipolar membranes. K+ ions were identified as being the predominant ions that electromigrate from the skim milk solution until pH 5.0 is reached, at which point its concentration becomes insufficient to counterbalance H+. The other cations partially replaced K+ to assure the electroneutrality of the solution, It appears that K+ is a necessary ionic species to ensure the best efficiency of the BMEA and to limit the non-desired migration of H+ across the CEM. Therefore, the electrical efficiency of BMEA is decreased by a loss of electrogenerated H+ due to a lack of sufficiently mobile ions such as potassium. Consequently, the enrichment of the skim milk with some potassium would be required in order to obtain a better electrical efficiency.