The existing modified fouling index (MFI0.45), based on cake filtration, uses a 0.45 mum microfiltration membrane to measure the particulate fouling potential of feedwater. To incorporate smaller colloidal particles into the MFI measurement, the application of ultrafiltration (UF) membranes was proposed. In this study, a suitable reference membrane for the MFI-UF test was investigated using polysulphone (PS) and polyacrylonitrile (PAN) UF membranes of 1-100 kDa molecular-weight-cut-off (MWCO) in tap water experiments. The stability of the MFI-UF value overtime and the influence of MWCO on the MFI-UF obtained were examined. Field emission scanning electron microscopy (FESEM) of the membrane surfaces and an analysis using the constant pressure blocking and cake filtration models were carried out. A stable MFI-UF* was found for PAN membranes. Whereas, the MFI-UF continuously decreased over time for PS membranes. The measured MFI-UF* (2000-13,300 s/l(2)) were significantly higher than the MFI0.45 expected for tap water, (1-5 s/l(2)). indicating smaller particles were retained. FESEM showed PAN membranes were homogeneously permeable. resulting in a stable MFI-UF* value as cake filtration was proven to be the dominant mechanism. Whereas, FESEM showed the 1, 2 and 5 kDa PS membranes were heterogeneously permeable and sparsely porous resulting in (i) artificially high MFI-UF* and (ii) limited cake filtration. Hence, the MFI-UF never stabilised. The MFI-UF* appeared MWCO independent within the 3-100 kDa range as most likely the cake itself acts as a second membrane, determining the size of particles retained and the resultant MFI-UF*.