- The formation of gypsum (CaSO4·2H2O) scale in ion-exchange membranes was investigated under a Donnan exchange regime. Two types of anion exchange membranes (AEM), homogeneous AMV and heterogeneous MA-40, were used for the study. Counter-ion flux, scaling-cation concentration in the fouled membrane and membrane potential served as indicators for membrane scaling. Locations of scaling domains were dependent on the membrane structure, and lead to different effects on the membrane performance. Scaling in the homogeneous membrane was mainly characterized by surface deposits, resulting in a moderate decrease of the counter-ion flux. On the other hand, the heterogeneous membrane showed a high degree of internal precipitation resulting in a complete clogging of the membrane, thus leading to earlier and more severe decline in counter-ion flux. Significant amounts of calcium were found within the fouled MA-40, supporting the other indications of internal scaling. The potential difference across the homogeneous AMV did not change significantly after operating under scaling conditions. However, for the MA-40, an increase of the membrane potential was observed, suggesting that CaSO4 scale blocks defects and imperfections in the heterogeneous matrix. Scaling experiments were conducted in a specially designed Donnan cell with optical apparatus for in-situ observation of scales. This allowed real-time observation of surface scaling on the ion exchange membranes. Gypsum growth centers were observed on the homogeneous AMV significantly earlier than flux decline appeared. This finding shows that it is feasible to detect onset of scaling on homogeneous anion-exchange membranes before the appearance of significant effects on membrane performance.