The influence of pH and calcium ion concentrations on the organic fouling of reverse osmosis (RO) membranes was investigated under two distinguished ionic strengths representing surface water (i.e., 10 mM) and seawater conditions (i.e., 600 mM). Variations in flux decline with respect to the feed water pH and calcium concentrations under these ionic environments were compared. Organic foulants deposited on the membrane surface were collected and characterized in terms of specific UVA (SUVA) and fluorescence excitation and emission matrix (FEEM) in conjunction with XAD-8/4 resin fractionation. Flux-decline curves obtained by various feed water pH and calcium concentrations under the low ionic strength condition were quite different with each other, while flux-decline curves obtained at the seawater-level ionic strength was almost identical regardless of variations in the feed water pH and calcium concentration. SUVA and FEEM results showed that the characteristics of organic matters attached on the membrane surface at high ionic strengths were significantly different from those at low ionic strengths. SUVA values and FEEM images indicated that organic foulants at the seawater-level ionic strength were mostly hydrophobic. Consequently, enhanced hydrophobic interaction dominantly controlled the rate and the extent of organic fouling at seawater-level ionic strength where the impacts of feed water pH and calcium were significantly masked. (C) 2010 Elsevier B.V. All rights reserved.