Presented organo-mineral membranes are the result of the addition of mineral fillers (Aerosil 200) to a solution of polysulfone (PSf) in N-methylpyrrolidone (NMP). They are prepared using the wet phase inversion process by casting the suspension on a glass plate and immersing it in water. The addition of aerosil to the casting solution, up to 2 vol.%, increases the resulting membrane permeability while maintaining their retentive properties. At a higher concentration of filler (3 vol.%), a decrease of the permeability and rejection properties as a function of operating pressure is observed. The effect of the filler concentration on the pore size distribution was examined using field emission scanning electron microscopy (FESEM) and the flow distribution through the Surface pores was calculated. Adding more filler to the solution changed the pore size distribution from a log-normal distribution to a bimodal distribution. At a certain filler concentration, the small amount of large skin pores influences the flow distribution through the membrane in such a way that the retention properties of the membranes decrease. The formation of the skin pores is explained by the presence of stress due to the amount of filler. The apparent permeability of the membranes is a result of the two opposite effects both initiated by the presence of the filler: an increase of the filler concentration enhances the surface porosity but slows down tl le membrane formation process that results in a thicker skinlayer. (C) 2000 Elsevier Science B.V. All rights reserved.