The performance of two hollow fiber ultrafiltration (UF) membranes, manufacturedfrom polyethersulphone (PES) with a polyvinylpirrolidone (PVP) coating, but withdifferent MWCO's (50 kDa and 150 kDa) were compared. Although, both the cleanwater flux (CWF) and the gross flux (GF) of the 50 and 150 kDa UF membranes werecomparable under the conditions employed, the net flux (NF) and water recovery(R) differed considerably. A net flux of 45 l/m 2 h was achieved with the 50 kDamembrane and a slow, steady decline in flux was observed. On the contrary, rapidflux decline was observed with the 150 kDa membrane and a backwash was requiredevery 3 mins, resulting in a net flux and product water recovery close to zero.The surface morphology of the membranes was investigated in an attempt to explainthe difference in the performance of the two membranes. Employing field emissionscanning electron microscopy, it appeared that while the surface porosity(7.3-8.7%) and pore size of both membranes were similar, the 150 kDa membraneshowed a relatively smooth surface, while the 50 kDa membrane revealed a muchrougher surface with a network of bridge-like structures around the pores. Thesuperior filtration performance of the 50 kDa membrane was believed to be due tothe highly interconnected pore system of this membrane. Includes 10 references, tables, figures.