Bipolar membrane electroacidification (BMEA) was used to produce high-purity bovine milk casein isolates. However, a deposit suggested to be a mix of calcium and magnesium salts was observed on the cation-exchange membrane (CEM) side in contact with the base. The aim of the present study was to complete the identification of the deposit formed on CMX membrane during bipolar membrane electroacidification of skim milk, and to characterize its physical structure at the interfaces of CEM. The results showed that a calcium deposit was found on both sides of the membranes as well as inside the membrane. This fouling was identified as calcium carbonate, but was likely a mix between calcium carbonate and hydroxyde.