The formation of CaCO3 deposits on RO and NF membranes was investigated in connection with permeate flux decline. Experiments, in the presence and absence of scale inhibitors, were carried out in a cross-flow membrane test cell in order to examine the extent of metastable region without scaling and the scale characteristics. Three types of flat sheet membranes (LPRO and NF) were used in the experiments. In the absence of inhibitors, scaling (as determined by observing scale crystals on the membrane surface in SEM micrographs) occurred at supersaturation ratio at the membrane surface higher than approximately 4 (or LSIm > 1). These experiments show that partial coverage of the membrane by scale does not necessarily result in a proportional flux decline. Furthermore, it appears that the type of membrane affects the dominant CaCO3 polymorph formed on the membrane surface. Experiments with several commercial inhibitors were carried out, at a supersaturation ratio at the membrane surface of 9.8 (or LSIm similar to 2), most of them with an inhibitor dose of 5 mg L-1. All inhibitors (but one) performed well, i.e. no significant flux decline was observed during these tests. However, SEM observations on dried membrane samples revealed that, for three inhibitors, a thin layer of sludge-type deposits was formed, totally covering the membrane surface. (C) 2007 Elsevier B.V. All rights reserved.