Significant uncertainty exists regarding incipient membrane scaling (a criterion for determining water recovery and antiscalant use) with negative impact on optimization of RO plant design and operation. Experimental data are reported on the initial phase of RO-membrane scaling by weakly supersaturated brackish water (calcite supersaturation ratio S similar to 1.9 to 3.1) in a narrow spacer-filled channel, simulating local (constant flux) conditions in spiral-wound membrane (SWM) modules. Scanning electron microscopy image analyses of numerous membrane samples, combined with complete physicochemical characterization of feedwater and retentate, provide new insights into early-phase membrane scaling. The unavoidable presence of small particles in feedwater (despite the use of tight cartridge filter) apparently promotes heterogeneous nucleation. Additionally, there is a clear evidence that, under these conditions, induction period for membrane scaling is practically non-existent. The initial deposit flux [mgCaCO(3)/(m(2) min)] on the membrane, obtained through measured crystal size distributions, exhibits strong dependence on supersaturation ratio; moreover, measured deposit fluxes are substantial and (if not mitigated) would be eventually detrimental to SWM performance. These data were obtained, while none of the frequently used parameters (bulk fluid turbidity, pH, Ca concentration, permeate flux) indicated particle/scale deposition on membranes. Implications of the new results, on practical and theoretical aspects of scaling, are discussed. (C) 2012 Elsevier B.V. All rights reserved.