The development of calcium sulfate dihydrate (gypsum) mineral scale in reverse osmosis (RO) membrane desalting was investigated by direct real-time observation of crystal growth. Gypsum scaling studies were conducted in a specially modified plate-and-frame reverse osmosis cell fitted with an optical window, with dark-field membrane lighting arrangement within the membrane cell to enhance crystal boundaries and allow recording of digital surface images magnified through an optical microscope. The evolution of the surface number density (SND) of gypsum crystals resembled a sigmoidal population growth process with an increasing rate of crystal formation at higher solution supersaturation (with respect to gypsum) at the membrane surface. The rate of formation of new crystals declined as the surface scale coverage increased, with surface scale progressing by diffusion-controlled growth of existing crystals as the gypsum crystal SND approached its plateau. The apparent crystallization induction time (i.e., observation time for the appearance of surface crystals) decreased with increased supersaturation, with single crystals growth reasonably characterized by a diffusion-growth model and a single crystal fluid mass transfer coefficient that was generalized by a Sherwood number type correlation. The present approach provides a convenient method of characterizing the kinetics of gypsum scale formation via quantification of single crystal mass transfer coefficient and evolution of the crystal SND. Such information can guide the development of comprehensive RO membrane scale formation models and characterization of mineral scaling propensities of RO membranes and source water where mineral salt scaling is of concern. (C) 2008 Elsevier B.V. All rights reserved.