Membranes were prepared from six samples of cellulose acetate (CA) differing in their average molecular weight (75-260 kg/mole) and molecular weight distribution using methyl acetate as solvent and 2-propanole as precipitant. The routes through the phase diagram and the evaporation times were varied in these experiments. Electron microscopy demonstrates that the amount of filter dust (CA particles deposited on the membrane surface) decreases as the fraction of low molecular weight material in the starting polymer becomes less. For low average molar mass of CA and moderate polymer concentrations in the casting solution the dust consists of individual spheres of relatively uniform size (1-2 mum) which cover the surface in a number of layers. With rising molar mass of the polymer the dust particles consist increasingly of strings of cohering and deformed beads of comparable size. Under these condition, some considerably smaller particles (less than or equal to0.1 mum) are also formed. Possible mechanisms of filter dust formation are discussed and measures for its prevention are proposed.