Hollow fiber ultrafiltration membranes with a corrugated outer microstructure were prepared from a PES/PVP blend. The effect of spinning parameters such as air gap, take-up speed, polymer dope viscosity and coagulation value on the microstructure and membrane characteristics was investigated. Fibers with maximum 89% surface area enhancement were prepared. The structured fibers and the round fibers spun under the same conditions had comparable (intrinsic) pure water permeability, molecular weight cut-off, pore size distribution and average skin layer thickness. This implies that the flow through the unit volume of the structured fibers will be enhanced compared to their round counterparts, while maintaining the same separation properties. A colloidal filtration method was used to determine the skin layer thickness. Structured fibers spun with a slow-coagulating polymer dope had varying skin thickness throughout the outer surface, which was dependent on the geometry of the fiber and was probably caused by varying local coagulation conditions around the structured outer surface of the fibers. A polymer dope with high coagulation value, on the other hand, resulted in a structured fiber with a homogeneous skin layer all along the surface.