This paper assesses the performance of a UF/RO demonstration plant located in the Oosterschelde estuary in the south-western part of the Netherlands. Spring blooms in the seawater pose a challenge to the plant because of the resulting increased fouling potential of the water. Determinations of the fouling indices SDI, SDI+ and MFI0.45 were carried out at the plant with different operational conditions, such as of coagulant addition and pH correction. Eight different membranes were used in the tests. In general, the UF performance was found to be good as the SDI values were around 1, provided standard membranes were used, and the MFI0.45 values lower than 1 s/L2. The MFI0.45 showed the same tendency as the SDI in most cases. As expected, whereas the SDI showed marked sensitivity to used membrane type and operational conditions, the SDI+ did not display this dependency and hence appear to be a more reliable fouling index than the SDI. Storing the RO feed overnight in the feed tank increased the fouling potential of the RO feed, likely caused by continued coagulation.