The interaction between membrane surfaces and solutes in solution plays an important role in determining the extent of membrane fouling. Fouling has normally been interpreted by the membrane-solute hydrophobic interactions but contradictory results have often been reported. In this study the extent of fouling was investigated in terms of the surface free energy change during the adsorption of foulants onto membranes. The free energy change of membrane fouling is derived as Delta G(f) = a + b root gamma(S)(d), where a and b depend on the surface properties of foulants present in aqueous solution. When three different membrane materials such as cellulosics, polysulfone, and fluoropolymer were tested, the fluoropolymer membrane gave the smallest fouling tendency, even though it is the most hydrophobic. The predicted fouling trend based on the dispersion component of surface tension was in better agreement with the experimental results rather than that based on the surface hydrophobicity.