Abstract New insights were gained by studying in detail (using Scanning Electrode Microscopy—SEM) incipient calcium-sulfate scaling of desalination membranes under realistic conditions; i.e. low supersaturation ratio S, once-through flow in narrow spacer-filled channels, with permeate-flux and cross-flow velocity levels encountered at the tail spiral-wound membrane (SWM) elements, where supersaturation first appears. Moreover, the feed-fluid was fully characterized, including its small-particle content. SEM images from desalination membranes and filters retaining small particles from feed- and retentate-samples were statistically analyzed. For 90min tests, elongated gypsum crystals are sparsely distributed throughout the membrane, at the smallest supersaturation; at elevated S, clusters of needles/rods and platelets develop with increasing membrane-surface density. Similarly, the initial scale deposition rate [mg/(m2min)] and membrane-surface coverage tend to increase rather sharply with increasing S. The data provide no evidence of induction period for membrane scaling, as also recently observed in studying incipient CaCO3 scaling. Additionally, there is strong indication that incipient CaSO4 scaling is due to growth of membrane surface particles (from nuclei developing right on the surface and/or transported to the membrane) and not due to deposition of crystals developing in the bulk. The implications of the new results are discussed and related R&D needs are outlined.