Fatty acid fouling of reverse osmosis (RU) membranes and its relation to adsorption under various chemistries had been investigated by using a crossflow RU filtration setup. Octanoic acid was used as model organic foulant. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy was utilized to semi-quantitatively determine the adsorption of the foulants on the membrane surface. The determined adsorption amount was related to the irreversible flux decline. Fouling was more significant at solution chemistries that resulted in larger adsorption amount, namely, lower pH, higher ionic strength and lower temperature. The flux decline increased first with the increasing ionic strength, and then it decreased at the ionic strength 50 mM. The rate and extent of flux decline decreased with the higher Ca2+ concentration due to the decreasing organic compound hydrophobicity. In the presence of magnesium, fouling behavior decreased slightly after the addition of magnesium ion, whereas, flux decline barely changed when Mg2+ concentration was increased gradually. To elucidate mechanisms of membrane fouling more clearly, the Lewis acid base interfacial free energy was also studied according to the extended Derjaguin-Landau-Verwey-Overbeek (extended DLVO) approach. (C) 2010 Elsevier B.V. All rights reserved.