The MFI-UF was developed to include smaller colloidal particles not measured in the existing Silt Density Index (SDI) and MFI0.45 fouling indices. This research investigates the application of the MFI-UF to measure and predict the particulate fouling potential of reverse osmosis (RO) feedwater and also to assess pretreatment efficiency. MFI-UF measurements were carried out at the IJssel Lake and River Rhine RO pilot plants of the influent feedwater, RO concentrate and after pretreatment processes. Pretreatment efficiency was compared based on MFI-UF, MFI0.45, and SDI measurements. The MFI-UF of the influent feedwater was approximately 700-2400 times higher than the corresponding MFI0.45 and SDI, due to the retention of smaller particles. A pretreatment efficiency of greater than or equal to80%, was found by the MFI-UF at both pilot plants. For the larger particles the MFI0.45 gave a 90-100% reduction. Minimum predicted run times for a 15% flux decline from MFI-UF measurements were shorter than that observed at the IJssel Lake pilot plant. This may be explained by problems with the ultrafiltration (UF) pretreatment at the time and/or almost negligible particle deposition in the RO pilot systems. Moreover, it was shown that cake resistance increased with ionic strength in MFI-UF tap water experiments and therefore, a correction of the MFI-UF index is required for salinity effects in RO concentrate. (C) 2003 Elsevier B.V. All rights reserved.