The axial development of calcium sulfate dihydrate (gypsum) scaling on selected reverse osmosis (RO) membrane surfaces was investigated experimentally in a plate-and-frame RO system. Scaling experiments with model solutions demonstrated progressive axial development of surface gypsum crystals along the membrane surface. The impact of surface crystallization was characterized via flux decline measurements and by both optical and high-resolution scanning electron microscopy (SEM) surface imaging. Surface coverage by gypsum scale (percent area covered and gypsum surface mass density) increased along the membranes (towards the membrane exit), consistent with the increase in concentration polarization. At the membrane channel entry, surface gypsum crystals were at their initial growth stage formed as primarily needle-like structures. With increasing axial distance from the entry, surface crystallization resulted in crystal structures that transitioned from the needle and plate-like structures, in the submicron size range, to partial rosettes to complete rosette structures in the mm size range. Significant differences were observed in the extent of surface scale coverage and surface crystal size among the membrane studied. Antiscalant addition to the feed solution led to progressive decline in the percent of area covered by scale with increasing antiscalant dosage and corresponding decrease in flux decline. Antiscalant addition resulted in perturbed surface crystal morphology of flattened and/or fused rosette crystal arms and to complete elimination of surface crystals at a sufficiently high antiscalant dosage. The present study demonstrates that gypsum scale development is affected by the formation of surface crystals on the membrane surface, thereby suggesting that there is merit to expanded research on the direct impact of surface topology and chemistry on surface crystallization of mineral salts. (c) 2005 Elsevier B.V. All rights reserved.