Acid whey (AW) is the main side-stream from acidified dairy products, known for its high contents of lactic acid and minerals. The present work investigates the effect of reverse osmosis (RO) pre-processing of AW (ROAW) and the current density during electrodialysis (ED) on the ED process performance. Firstly, the limiting current density of the ED system when treating AW and ROAW was determined. Then, the AW and ROAW have been demineralized and deacidified at under-limiting or over-limiting current density batch operating conditions. RO pre-processing of AW and current density strongly affect the demineralization rate and solutes mass transport across the membranes. Lactate transport can be rationalized by the weak acid dissociation mechanism, while water splitting played a minor role in the ED process performance. Among cations, calcium and magnesium were most affected by RO pre-processing, due to their high affinity to the cation exchange membranes. Lactose and galactose were found in small amounts in the concentrate solution, while no glucose was detected for most conditions even though the glucose content decreased in the diluates. The energy consumption was up to 2 fold higher for ROAW compared to AW