Presented organo-mineral membranes are the result of the addition of Aerosil 200 spheres 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. For the first time, transparent suspensions are used to investigate the effect of the filler on the membrane formation process. It allows the transport of nonsolvent into the membrane to be followed by light microscopy. The results prove that an increase of the filler concentration slows down the indiffusion rate. Cloud point measurements indicate that the addition of fillers changes the thermodynamic behavior of the polymer/solvent/nonsolvent system in such a way that demixing occurs at a lower concentration of nonsolvent. A special experimental 'slide'-setup is designed and presented here to follow the instantaneous demixing process of the filled polymer solutions as a function of time. These experiments prove that the presence of a small amount of filler increases the demixing rate in the initial stage of membrane formation. A further increase of the amount of filler decreases the demixing rate of the instantaneous demixing process due to viscosity effects, (C) 2000 Elsevier Science B.V. All rights reserved.