Protein-containing waste liquors from the manufacture of soy protein concentrates were processed by ultrafiltration to recover soluble protein using three different membranes (with molecular weight cut-offs of 10, 30 and 50 kDa). Operating at 20 degrees C under reversible conditions, the experimental data of the normalized permeate flux (NPF) obtained at various transmembrane pressures were well described by a model reported in the literature. For each membrane and transmembrane pressure, the values of the parameters involved in the model were calculated. Operating at selected transmembrane pressures, protein rejections of 0.705, 0.747 and 0.637 were determined for the 10, 30 and 50 kDa membranes, respectively. Operation with the 10 kDa membrane at temperatures in the range 30-50 degrees C and operation with the 30 or 50 kDa membranes at 40 or 50 degrees C resulted in hysteresis. (c) 2006 Society of Chemical Industry.