The aim of this paper is to demonstrate the factors influencing a protein filtration process. Therefore single and mixed solutions of lysozyme and myoglobin were ultrafiltered through a hydrophilic membrane (cellulose acetate type) with a molecular weight cutoff (MWCO) of 30 kDa. The proteins are of similar molar mass and size. Filtration was done using a thin spiral channel module (TCF10). The change in transmission and flux was evaluated and the data of the single protein solutions compared with the binary protein mixture. Further, the effects of Dean vortex formation, was evaluated. From the collected data the effects of the parameters dominating the filtration process were assessed. By comparing the experimentally obtained mass transfer coefficients with literature data, it can be seen that the curved channel led to an increased mass transfer as compared with classical modules. For both proteins, a Sherwood correlation was derived and based on these data a separation process was modeled. Results obtained from separation experiments correlated well with the developed model. It could be shown that the separation of proteins by ultrafiltration is possible, but for proteins bearing similar molar masses it is a difficult task. Separation factors up to 2.5 were achieved. As expected the selectivity increases with the crossflow velocity and decreasing transmembrane pressure (TMP). The separation process requires careful choice of the pH value and the ionic strength. Further, attention has to be paid to the membrane stability and protein solubility while varying the pH value. (C) 2003 Elsevier B.V. All rights reserved.