Flat sheet Zirfon(R) membranes composed of organic (polysulfone; PSf) and inorganic (zirconium oxide; ZrO2) constituents were prepared by the phase-inversion process. The casting solutions consist of 18 wt% polymer solutions of PSf in N-methyl-pyrrolidone to which different amounts of ZrO2 were added. The membranes were evaluated by measurements of their permeability, porosity and dextran retention. High resolution scanning electron microscopy was applied to obtain direct information on the skin morphology. The membrane permeability was found to increase for growing amounts of inorganic grains added to the casting solution. However, image analysis of the surface micrographs did not reveal corresponding changes in the surface porosity and pore dimensions of the skin. The observed flux behaviour is attributed to the disturbance of the normal phase-inversion process by the presence of the inorganic grains. From 40 wt% ZrO2, the hindrance by the inorganic grains is sufficiently high to cause the formation of a highly porous surface layer. A further increase in wt% ZrO2 influences the formation of the total skin layer, enhancing a higher throughput to the porous sub layer.