Current fouling indices typically employed in RO practices, such as silt density index (SDI) and modified fouling index (MFI), have suffered greatly from their inability to predict actual fouling potential primarily due to the erroneous interpretation of fouling mechanisms. Our findings clearly demonstrated that the effect of pore blocking should be excluded during fouling index measurements to simulate real RO applications. Thus, new concept of cake fouling index (CFI) was developed in order to accurately evaluate true fouling cake layer resistance. Specifically, the CFI was determined through consecutive filtration tests by subtracting the flux decline of the secondary filtration from that of the first one to eliminate the effect of pore blocking. The results proved that CFI better predicts the degree of fouling rate in RO experiments than MFI. It was also revealed that it could be utilized as a useful tool for identifying and evaluating the fouling mechanisms. Through a comparison of MFI and CFI, it was shown that pore blocking was enhanced as much as cake formation when pH decreased, while divalent cations (Ca2+) increased only cake formation on the membrane surface. This newly developed index refining existing MFl method is expected to provide more precise information about RO membrane fouling, especially for the design of effective pretreatment processes. (C) 2015 Elsevier B.V. All rights reserved.