Destabilization-flocculation is often used as a pretreatment before ultrafiltration of drinking water and secondary wastewater effluent. The fouling of ultrafiltration membranes by colloidal iron hydroxide-oxide has been studied by measuring the pore streaming potential of a series polysulfone type OF membranes with 10 KDa-50 KDa NMWCO. SEM-EDX was carried out on the fouled membranes and the iron distribution in and on the fouled membrane was determined. Fouling of the membrane as measured by flux reduction was usually accompanied by a positive change in zeta potential and iso-electric point (IEP) of the membrane. Severity of fouling seems to correlate with the presence of some pores of size larger than part of the iron colloid particle size distribution. SEM-EDX confirmed presence of iron within those membranes (20 KDa) suffering the most fouling. An initially large change in zeta potential was seen even after relatively small amounts of solution were filtered through the membrane. A control experiment showed this is not due to iron adsorption equilibrium, but should probably be attributed to fouling.