The fouling on the ion exchange membrane (IEM) was investigated every 8 h during the desalination of the high CaSO4 supersaturated water by electrodialysis. Only the cation-exchange membrane (CEM) was seriously scaled in the experiments. On the CEM, no fouling was visually observed but rhombohedral crystals that were identified as calcite by XRD and EDS emerged under SEM after 24 h due to high alkalinity in the solution. During the experiments without filter, needle and rosette structures of the gypsum scale formed by surface crystallization were firstly observed at the corner of the fluid channel after 32 h, and it developed rapidly on the CEM due to bulk crystallization subsequently. However, the fouling on the CEM was not dominated by crystal precipitations from the bulk solution in the experiments with filter. The gypsum fouling developed from sparsity to dense colonies along the fluid flow direction. The fouling was characterized by needlelike shape and rod clusters that emanated from the membrane surface from the inlet to the middle of the fluid channel, but it grew into agglomerate fouling due to the bulk crystallization near the outlet of the fluid channel. The gypsum fouling could augment significantly the surface resistance of the IEM and that may be irreversible. (C) 2010 Elsevier B.V. All rights reserved.