The aqueous extract of soy flour is an emulsion/suspension of proteins, lipids and carbohydrates. The foulant deposit formed on the surface of polysulfone membranes in the ultrafiltration of this complex extract was investigated from several aspects including thickness, physical structure, chemical analysis and rheological behaviour. SEM studies showed the thickness of the foulant deposit was approximately 0.2 mu m for 50000 MWCO membrane and 0.4 mu m for 100000 MWCO membrane. The structure of the foulant deposit consisted of lipids in a globular form of 0.2 to 1 mu m diameter adhered to, and supported by, a protein-polysaccharide matrix. Rheological measurements were conducted on a sample of the foulant deposit collected from the 100000 MWCO membrane. This foulant deposit exhibited pseudoplastic and viscoelastic properties which totally resisted the surface shear stresses in the flat-plate module. Recovery of the water flux of the fouled membranes was achieved by a four-stage cleaning procedure comprising successive stages of washing with sodium hydroxide, protease detergent, sodium hypochlorite and flushing with water.